Response of deep-water fore-arc systems to sea-level changes, tectonic activity and volcaniclastic input in Central America

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Spec. Pubis. int. Ass. Sediment. (1991) 12. 273-292

Response of deep-water fore-arc systems to sea-level changes, tectonic activity and
                    volcaniclastic input in Central America

                        JUTTA WINSEMANN alld HARTMUT SEYFRIED
   btstitut fur Geologie ulld Paliiolltologie. Ulliversitiit Stuttgart. Boblillger Strasse 72. D-7000 Stuttgart I.
                                                      Germany

                                                      ABSTRACT

         The incipient island-arc system of southern Central America (Cretaceous - early Oligoccne) is character-
         ized by thick turbidite systems. which mainly filled inner fore-arc troughs. Outcrop data show four.
         second-order depositional sequences in the deep-water sedlments. The formation of these depositional
         sequences is strongly related to the morphotectonic evolution of the island-arc system. Each depositional
         sequence reflects the complex interaction between global sea-level fluctuations. sediment supply. and
         tectonic activity.
             Strong marginal uplift and high volcaniclastic sediment supply dunng early to late Paleocene and late
         Eocene times caused the formation of coarse-grained channel-lobe systems. During late Paleocene and
         mid-Eocene times. fine-grained. thin-bedded turbidite systems were deposited. oWing to regIOnal
         subsidence and a decrease in volcanic supply. Uplift and subsidence of sediment-source areas acted as
         major controls on deposition of basinal cycles.

                                                  INTRODUCTION

The fore-arc area of southern Central America be-                 careous turbidites and slump deposits preVailed in
longs to the middle America arc-trench system,                    the outer fore-arc basins. The shape of these sedI-
which extends along southern Mexico and the                       mentary basins was probably controlled by a major
Central American isthmus. The Cocos plate is be-                  strike-slip system, which transected the island arc
ing subducted beneath the North American and                      perpendicularly in the region of central Costa Rica.
Caribbean plates (Burbach et al., 1984) and is cur-               A broader discussion of the regional geologIcal set-
rently being subducted beneath Costa Rica at 9 cm!                ting is given in the chapter by Seyfried et al.
yr (see Fig. 1 in paper by Seyfried et al.)                          Because modern and ancient submarine fans show
  Subduction and related processes in southern                    great variability in shape and facies relationship,
Central America started in Late Cretaceous times.                 the type and ttmlng of turbidite events have been
The incipient island-arc system is characterized by               discussed recently by many authors with regard
narrow structural basins in the inner fore-arc area,              to sediment supply, tectonic activity, and sea-
an outer arc, and a trench slope system (Fig. 1).                 level fluctuation (Barnes & Normark, 1985; Mutti,
  The sediments of the ophiolitic basement (Nicoya                 1985; Shanmugam et al., 1985; Stow et al., 1985;
Complex) are mainly pelagic deposits and consist of                Mutti & Normark, 1987; Kolla & Macurda, 1988;
radiolarian cherts, black shales, and deep-water                   Shanmugam & Moiola, 1988). Classifications of
limestones of Late Jurassic to Late Cretaceous age                 submarine fans have been compiled, based on theIr
(Schmidt-Effing, 1979; Lundberg, 1982; Baum-                       tectonic setting (Mutti & Normark, 1987;
gartner et al. 1984; Gursky, 1984; Astorga, 1987).                 Shanmugam & Moiola, 1988).
   From Maastrichtian to Eocene times, thick suc-                     According to the sequence-stratigraphic model,
cessions of volcanic1astic turbidites were deposited               the development of submarine fans is mamly related
in the inner fore-arc basins, while thin-bedded. cal-              to global sea-level lowstands and they form part of

                                                            273
274                                     1. Winsemann and H. Seyfried

                                                                               Fig. 1. Tcctonic setting and location
                                                                               map of Upper Crctaceous to upper
                                                                               Eoccne turbiditc systems of Ihe
                                                                               forc-arc area of Costa RIca and
                                                                               southwest Nicaragua (modified after
                                                                               Astorga et al., 1989). Inner fore-arc
                                                                               basins: (1) Nicaragua Trough
                                                                               (A, southwest Nicaragua; B. Bahl3
                                                                               Sahnas); (2) Tempisque Trough
                                                                               (C, Barbudal; D, Curu). Outer
                                                                               fore-arc basins: E, Samara Basin; F.
                                                                               Cabo Blanco Basin; G, Quepos
                                                                               Basin. For location of figure within
                                                                               Central America, compare with
                                                                               Fig. 2 of the paper by Seyfried et al..
                                                                               this volume.

the lowstand systems tract. This lowstand systems         who distinguished three types of turbidite system,
tract can be divided into a lowstand fan (basin-floor     type I, type 11, and type III (Fig. 2), which more
fan) and a lowstand wedge (Posamentier & Vail,            recently were related to the long-term stability of
1988). The lowstand fan is characterized by the           basins (Mutti & Normark, 1987). Active-margin
deposition of sand lobes, whereas the subsequent          fans are considered usually to be small, coarse-
lowstand wedge is dominated by channel-fill deposits      grained systems, owing to proximal sources, narrow
and finer grained, wedge-shaped slope deposits. The       shelves, and coastal plains (Mutti & Normark, 1987;
lowstand wedge has also been described as slope           Shanmugam & Moiola, 1988). However, in the study
fan, slope-front fill, and wedge and cone (Mitchum,       area, small, sand-rich fans only occur in lower Paleo-
 1985; Vail & Sangree, 1988). In contrast, carbonate      cene and upper Eocene deposits. whereas fine-
platform environments suggest that the sedimentary        grained, thin-bedded turbidite systems formed from
response to relative sea-level changes in these           late Paleocene to mid-Eocene times.
systems is opposite to that observed in siliciclastic
systems (Sarg, 1988; Dolan, 1989). During high-
stands, mud- and sand-size carbonate sediment is           DEPOSITIONAL SEQUENCES IN THE
shed off the platform and transported into the                INNER FORE-ARC TROUGHS
adjacent basin, whereas conversely, during sea-level
lowstands, the production capability of shallow water     The inner fore-arc area comprises two major basin-
carbonate is strongly reduced and subaerial exposure      fill systems: (1) Upper Cretaceous to upper Eocene
of platforms may result in meteoric cementation and       turbidite systems of the Nicaragua Trough and (2)
karstification (Sarg. 1988). In hybrid siliciclastic-     Upper Cretaceous to upper Eocene turbidite systems
carbonate basinal cycles, carbonate deposition            of the Tempisque Trough (Fig. 1).
therefore reflects conditions of relative highstands of       Outcrop data show four second-order depositional
sea level, whereas lowstands of sea level are docu-       sequences in the deep-water sediments, which range
mented by siliciclastlc sedimentation. In contrast,       considerably in size, geometry and location. Each
small to moderate falls of sea level in ramp settings     depositional sequence is characterized by a particular
may expose only the inner part of the ramp, resulting     association of turbidite systems, which reflect the
in lowstand deposition of mixed siliciclastic-            prevailing local conditions of sediment supply, tec-
allodapie carbonate sedlments (Dolan. 1989).              tonic activity, and global sea-level fluctuations during
   Another model was presented by Multi (1985).           Late Cretaceous to Eocene times (Fig. 3).
Fore-arc deep-water response                                                          275

                                        SAND DEPOSITION IS MAINLY                                              TYPE   m
                                        RESTRICTED TO    CHANNEL~FILL
                                        SEOUENCES                                                 /'
                                                                                   /-         %

                                        SAND IS DEPOSITED IN BOTH                                              TYPE n
                                        CHANNEl· FILL AND LOBE                       ~
                                        SEOUENCES

                                                               ~
                                        SAND   I~   PREOOMINAN flV                                             TYPE (
                                        DEPOSITED IN NON-
                                        CHANNElIZED LOBES

                                                                                                                        A

                                      LOWSTAND SYSTEMS TRACT
                                                                                                                  WEDGE

Fig. 2. Comparison of Multi's
                                                                              CHANNEL SANDS
depositional systems and the                              OVERBANK DEPOSITS
lowstand systems tract of the             BASIN flOOR
                                           TIJlBIDIT                                                   BASIN FLOOR FAN
sequence-strati graphic model. The
type I turbidite system corresponds
to the basin-floor fan, the type 11
and type III sy~tems correspond to
the lowstand wedge (also called           SHEET LOBE FACES
slope fan or prograding wedge).
(A) Simplified after Multi &
 Normark (1987). (8) Simplified                                                                                         B
after Vail & Sangree (1988).

Nicaragua Trough                                               ness decreases strongly to the south and                 i~   only
                                                               about 3000m in section B.
The deposits of the Nicaragua Trough are mainly
exposed along the Pacific coast of southwest
Nicaragua and northern Costa Rica and were re-
                                                               SectlOlI A
corded in two major sections (A, B, Fig. I).
   Large-scale subsidence of the southern basin mar-           In southwe~t Nicaragua. Upper Cretaceou~ to Palco-
gin is recorded by thickness variations and facies             cene rocks crop out only in poor inland cxpO\urc\
development of turbidite systems from Late ere-                and are mainly recorded from well data (Anony-
aceous to Eocene times. The greatest thickness of              mous. 1972. cited in Weyl. 1980). According to
the basinal sequence occurs in the northern part of            Wey1 (1980) these deposits mainly con~l~t of about
the Nicaragua Trough (section A), where almost                 3500 m of slhciclastic turoidltcs and tuffaceou~
6000 m of deep-water sediments are recorded. Thick-            shales.
A                                     B                                         c                      o                                                           N

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                                                                                                                                                              mudstones

                                                                                                                                                    ~ pelagoc limestones
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Fig. 3. Bm.m-wlde correlallon of dep
Fore-arc deep-water respollse                                      277

   Large-scale slump and cobbly mudstone facies in      each eycle, the channel overbank systems may con-
the uppermost Cretaceous section indicate a slope       sist of one or several nested channels that formed as
or base of slope environment. Cobbly mudstones          a result of channel avulsion.
contain c1asts of well-rounded basalts, radiolarites,      The channelized deposits are characterized by
and sandstones, indicating erosion of upheaved ba-      different facies:
saltic islands (see, for comparison, the chapter by     1 Basinal minor channelized sand lobes, very rich in
Seyfried et al.). Locally, this slump apron is over-    isolated larger Foraminifera and other shelf-derived
lain by volcanic and carbonate breccias of early        fossils.
Paleocene age (Weyl, 1980; Hofherr, 1983; Calvo,        2 Channel-fill deposits rich in resedimented neritic
1987; Sprechmann et al., 1987). The volcanic breccias   carbonates and bioclasts of larger Foraminifera,
are up to 50 m thick and consist of dominantly          corals, molluscs, and echinoids.
pebble-sized angular volcanic rock fragments and        3 Very coarse-grained channel-fill deposits, which
minor constituents of neritic carbonates, hemipelagic   consist of volcaniclastic conglomerates and breccias
calcilutites, and cherts (Hofherr, 1983). Up section,   with boulders up to 4 m across. Debris flow deposits
these volcanic breccias are overlain by carbonate       and high-density turbidites contain crystal-rich, juv-
megabreccias with huge blocks of shallow-water          enile volcanic detritus, indicating rapid redeposition
limestones, which reach maximum diametres of            of subaerial pyroclastic flows by subaqueous debris
about 50m (Calvo, 1987). The matrix is made up of       flows or high-density turbidIty currents.
calcilutites and volcanic detritus, most probably de-   4 Sand-rich channel-fill deposits, which are associ-
rived from reworking of periplatform deposits and       ated with overbank complexes and thick overbank
erosion of the underlying strata. According to Calvo    wedges. These sand-rich deposits consist of
(1987), the neritic carbonates represent allodapic      amalgamated sand bodies with very common loading
platform carbonates of Late Cretaceous age and are      and dewatering structures and overlie the coarser
similar to the Costa Rican 'Barra Honda' platform       grained volcamclastic conglomerates and/or carbon-
carbonates (see chapter by Seyfried et al.). Fora-      ate breccias. Channel-associated overbank deposits
miniferal biostratigraphy indicates that initial al-    consist of fine-grained, thin-bedded turbidities, which
lodapic limestone deposition occurred during early       may be intensively current laminated. The overlying
Paleocene times (Plc/P2; Sprechmann et al., 1987).      overbank wedges are characterized by thicker
Upper Paleocene to middle Eocene deposits are            bedded turbidites with common progradational
poorly exposed and mainly consist of thin to me-        cycles.
dium-bedded turbidites. A lobe complex, which
occurs within these sedimentary units, probably indi-
                                                        Sectioll B
cates the transition from early to middle Eocene
deposits, but no biostratigraphic data are available.   An almost complete section of Upper Cretaceous to
These lobe deposits are up to 80 m thick and consists   upper Eocene deposits crops out along the north-
of thick-bedded to massive, amalgamated sandstones,     western Pacific coast of Costa Rica. These sedi-
which are rich in volcanic rock fragments. Up sec-      mentary units were depoSited close to the southern
tion, they are overlain by 300 m of thin-bedded         basm margin of the Nicaragua Trough and overlie a
volcanic1astic turbidites of probably middle Eocene     pre-Santoman basement (Baumgartner et al., 1984;
age (Weyl, 1980; Astorga, 1987).                        see chapter by Seyfned et al.).
   The upper Eocene deposits are well exposed along        Sedimentation started with rock-fall brecclas of
 the Pacific coast and consist of multicyc1e fan com-   late Campanian age (Baumgartner et al., 1984).
plexes. This sedimentary unit shows three cycles        These breccias are T1ch m resedimented neritlc
ranging in thickness from about 250 to 600 m and is     carbonate clasts and pass up-sectIOn mto pebbly
characterized by well-developed channel overbank        sandstones that contain large amounts of larger
systems. Each cycle consists of a basinal unit of       Foraminifera. The breccias arc overlain by 80 m of
channelized conglomerates/sandstones, overlain by       interbedded hemipelagic limestones and thin-bed-
an mterval of thin-bedded turbidites, interpreted as    ded sandstones. The siliciclastlc influx increases up
channel-associated overbank systems and overbank        section and sandstone beds become thicker. Unchan-
 wedges (Mutti & Normark, 1987). The channelized        nelized, thick-bedded turbidite~ abruptly overlie the
facies are commonly characterized by an upward          calcareous unit. These lobe deposIls consist of two
decrease in gram size and bed thickness. Within         major progradauonal cycles, separated by a small
278                                     1. Wi1lsema1l1l a1ld H. Seyfried

interval of thin to mediun-bedded turbidites. Up          exposed. The sequence boundary may be indicated
section the thick-bedded turbidites gradually pass        by the occurrence of large-scale slump features and
into siliceous limestones and mudstones of late           cobbly mudstone facies. However, there is no age
Paleocene to early Eocene age (Baumgartner et al.,        control and debris aprons and slides have been
1984). The siliceous limestones are rich in Radiolaria    reported also from highstand deposits and therefore
and planktonic Foraminifera and are similar to sil-       may form at any time independent of the position of
iceous limestones, which formed simultaneously in         sea level (Weimer, 1990). The deposition of the
the outer fore-arc region of Samara (section E) and       volcanic and carbonate megabreccias during early
Cabo Blanco (section F). The middle Eocene de-            Paleocene times is attributed to a major volcanicl
posits consist of thin to medium-bedded turbidites,       tectonic event that affected the southeastern basin
rich in volcanic ash. The transition from the under-      margin. A previous interpretation of the em-
lying lower Eocene deposits is gradational. Owing to      placement of the carbonate megabreccias has been
the poor exposures of these deposits, no further data     given by Calvo (1987), that these blocks detached
are available.                                            from steep submarine fault-scarps of the Hess
   The upper Eocene deposits make up the greatest         Escarpment. However, the association with volcanic
part of the sedimentary record. These sedimentary         breccias indicates that, most probably, volcanic ac-
units consist, as in the northern basin area, of multi-   tivity and accompanying basin-margin fault activity
cycle fan complexes and are characterized by well-        resulted in the break up of carbonate shelf rocks,
developed channel overbank systems. Two cycles            which subsequently were transported into the basin.
are recorded, ranging in thickness from about 600 to         Seque1lce 2 is only exposed in section B. The
700 m. As in section A, each cycle consists of a basal    sequence boundary between sequence 1 and se-
unit of channelized conglomerates/sandstones, over-       quence 2 most probably developed during late
lain by an interval of thin-bedded turbidites.            Paleocene times and is indicated by the onset of a
                                                          new progradationallobe complex. These sand lobes
                                                          are interpreted to be part of a new lowstand systems
/merprelalio1l
                                                          tract (Posamentier & Vail, 1988). Up section the
In the Nicaragua Trough, four sccond-order depo-          sandstones gradually pass into siliceous limestones
sitional sequences have been identified. Two third-        and mudstones of late Paleocene to early Eocene
order depositional sequences could be determined           age, indicating a rapid sea-level rise with a drastic
within the upper Eocene sequence of section A.             decrease in siliciclastic sediment supply. These de-
   Seque1lce 1 is mainly exposed in the southern           posits are interpreted to be part of the transgressive
Nicaragua Trough (section B) and consists en-              and highstand systems tracts.
tirely of unchannelized lobe deposits. The sequence           Seque1lce 3 mainly consists of thin to medium-bed-
boundary is indicated by the abrupt onset of sand          ded, ash-rich turbidites. In section B, the sequence
lobes during the latest Maastrichtian at approxi-          boundary between sequence 2 and sequence 3 is
mately the 68 Ma sea-level fall (Haq et al., 1988).        indicated by the transition from siliceous limestones
The sand lobes are interpreted to be part of the           to siliciclastic turbidites, which occurred during the
lowstand systems tract and to have formed during a         middle Eocene (Baumgartner et al., 1984) and may
rapid fall of sea level when sediment excavated from       be correlatable with the 49.5 Ma sea-level fall (Haq
the slope and shelf were transported via submarine         et al., 1988). Progradational and retrogradational
canyons into deep water (Posamentier & Vail, 1988).        cycles within these sequences probably indicate low-
The small thicknesses of these lobe sediments indi-        stand, transgressive, and high stand deposition.
cate that they probably were deposited close to a             In the northern basin area, the sequence boundary
structural high, which deflected the bulk of the           is indicated by the formation of unchannelized lobe
sediment from this area. The interval of thin to           deposits, which overlie thin to medium-bedded tur-
medium-bedded turbidites are interpreted to have           bidites. These lobe deposits are interpreted as
formed during the late lowstand systems tract (Vail        the lowstand systems tract. The overlying thin to
& Sangree, \988). Condensed sections of the trans-         medium-bedded volcaniclastic turbidites most prob-
gressive/highstand systems tracts could not be deter-      bly form part of the transgressive and highstand
mined and most probably were eroded during a               systems tracts, but no age control has been obtained.
subsequent fall of sea level (cf. Weimer, 1990).              Seque1lce 4 is characterized by multicycle fan
    In the northern basin area, sequence \ is poorly       complexes with channelized lobes, very coarse-
Fore-arc deep-water response                                       279

grained channel-fill deposits. and thick overbank
                                                           Section C
wedges. The sequence boundary between sequences
3 and 4 is indicated by the abrupt onset of coarse-        The oldest sediments of the Barbudal area consist
grained. channelized deposits over thin-bedded             of tuffs and siliceous limestones of probable early
turbidites. reflecting initiation of erosion and a deep-   Campanian age (Seyfried & Sprechmann. 1986). Up
marine expression of a type 1 sequence boundary            section. coarse-grained basaltic breccias and con-
(Posamentier & Vail. 1988).                                glomerates occur. The basal unit of these breccias
   Cycles most probably developed in response to           consists of weathered basalt rubble and gravel. indi-
third-order eustatic sea-level changes and are             cating erosion of basaltic islands (see paper by
interpreted as cyclic stacks of individual lowstand        Seyfried et al.). The upper units contain less-altered.
systems tracts. consisting dominantly of the lowstand      probably cliff-derived basalt detritus and show
wedge (Posamentier & Vail. 1988).                          intercalations of resedimented rudist debris. echin-
   The high content of larger Foraminifera within          oids. oysters. corals. and larger Foraminifera.
channelized lobe deposits most probably reflects the       Resedimentation of these breccias occurred
resedimentation of carbonate shoals during the in-         during the late Campanian (UCII-12; Seyfried &
itial fall of sea level at the shelf margin (Shanmugam     Sprechmann. 1986). The breccias are overlain by a
et al .• 1985). Exposure of arc-fringing reefs or car-     unit of thin to medium-bedded mudstones. which
bonate shelfs during the continuous fall of sea level      reach a thickness of 730 m and contain two inter-
led to the destruction and resedimentation of these        calations of calcarenite beds. rich in neritic fossils.
systems into the basin. forming allodapic carbon-          Deposition of these dominantly fine-grained facies
ate breccias. which commonly were deposited in             persisted from terminal late Campanian until early
channels.                                                  Paleocene times (Rivier. 1983). Up section. 200 m of
   Deposition of very coarse-grained channel-fill de-      carbonate breccias and calcarenites occur. These
posits of the lowstand systems tract occurred when         breccias and calcarenites form small debris apron-
major tectonic/volcanic pulses increased the supply        like features and were deposited in early Paleocene
of siliciclastic detritus and submarine canyons were       times (P2/P3A; Rivier, 1983). Deposition of the
deeply incised into the small shelves and/or volcanic      carbonate sediments terminated abruptly with the
flanks. Neritic carbonate blocks up to tOm across          onset of coarse-grained channelized conglomerates
within channels suggest the repeated destruction of        and sandstones during late Paleocene times
basin-margin areas. Channel-overbank systems               (P4; Rivier, 1983). These channel-fill deposits consist
formed when lowstand deltas began to prograde              of coarse-grained volcaniclastic conglomerates and
during the period of lowstand wedge deposition. As         are commonly not associated with overbank de-
the depositional slope of the delta became over-           posits. Up section they gradually pass into finer
steepened and unstable. sediment failure began and         grained sandstones and calcareous mudstones. The
large amounts of fine-grained sediments were trans-        transition from coarse-grained channelized deposits
ported into the basin. forming extensive channel           to the fine-grained calcareous turbidites occurred
overbank systems and overbank wedges. Sections of          during the latest Paleocene (Rivier. 1983) and the
 the transgressive/highstand systems tract could not       lower and central parts of this fine-grained facies
be determined and were most probably eroded as             have been dated as early Eocene (Rivier, 1983). Up
sea level began to drop again and erosion and rapid        section the siliciclastic influx markedly increased and
sedimentation occurred in the deep water (cf.              minor lobe systems formed. These lobe deposits
 Weimar. 1990).                                            consist of thin to medium-bedded sandstones with
                                                           intercalated ash beds and pebble-sized volcanic
                                                           conglomerates. Fragments of larger Foraminifera
                                                           are commonly observable. The onset of these
Tempisque Trough
                                                           dominantly volcaniclastic deposits most prob-
The deposits of the Tempisque Trough are mainly            ably occurred during the early middle Eocene
recorded from the Barbudal area (section C) and the        (A. Astorga. personal communication, 1990).
Curn area (section D) where a sedimentary unit of
almost 3500 m is recorded. The development of se-
                                                           Section D
quences within this basin is strongly influenced by
local parameters of volcanic and tectonic activity.        In the Curn area, sedimentation started in the early
280                                     J. Winsemallll and H. Seyfried

Campanian with siliceous mudstones and beds of            times in this area. In addition, palaeocurrent data
volcanic ash, which unconformably overlie the             suggest deposition by axial flows down an elongate
ophiolitic basement or pre-Campanian deep-water           basin, most probably supporting the development of
sediments (Schmidt-Effing, 1979; Lundberg, 1982;          lobe deposits. The overlying thin-bedded turbidites
Astorga, 1987). Intercalated channeIized debris-flow      and shales are interpreted to have been formed
deposits consist of resedimented hemipelagic mud-         during the late lowstand systems tract and probably
stones, locally derived neritic fossils, and minor con-   arc also parts of the transgressive and highstand
tents of recycled material from the Nicoya Complex.       systems tracts. Condensed sections could not be
Upwards, Foraminifera-rich calcareous mudstones           identified.
occur. These limestones are late Campanian in age              In the northern basin area, thick lobe deposits are
(Calcarata zone, Schmidt-Effing, 1979) and grade          absent and instead mainly fine-grained mudstones
up-section through argillaceous limestones into thin-     were deposited. The sequence boundary may be
bedded turbidites, reflecting an increasing sediment      indicated by a thick calcarenite bed, rich in neritic
input during the early Maastrichtian. These inter-        fossils, reflecting a relative lowstand of sea level with
bedded sandstone and mudstone facies are overlain         erosion of shelfal sediments. Deposition of this cal-
by thick lobe systems, which initially formed in the      carenite bed occurred during the latest Maastrichtian
latest Maastrichtian and persisted until the late         (Rivier, 1983) at approximately the 68 Ma sea-level
Paleocene (Lundberg, 1982; Baumgartner et al.,            fall (Haq et al., 1988). A major volcanic/tectonic
1984). The sand lobes consist of thick-bedded to          event is indicated by the formation of carbonate
massive sandstones, which are up to 12 m thick and         breccias and calcarenites on top of the sequence.
interbedded with thin-bedded turbidites and shales.        According to the biostratigraphic data, deposition of
Two major progradational cycles are separated by a         these allodapic limestones could have been con-
larger interval of thin-bedded turbidites and shales.      trolled also by the brief eustatic drop that occurred
The lobe deposits are overlain by fine-grained, thin-      at the 63 Ma sea-level fall (Haq et al., 1988). How-
bedded turbidite systems, which probably were de-          ever, the 63 Ma sea-level fall is also related to major
posited during latest Paleocene and early Eocene           global tectonic events (Schwan, 1980), and the high
times (Astorga, 1987). During middle and late              content of volcanic detritus calls rather for a major
 Eocene times, thick amalgamated sandstone com-            control on deposition by volcanic events.
 plexes are absent and instead medium to thick-                 In the southern Tempisque Trough, the boundary
 bedded turbidites developed, usually forming minor        between sequence 1 and sequence 2 is indicated by
 thickening and coarsening upward cycles. These            the onset of a new progradational lobe complex.
 lobe systems are rich in larger Foraminifera and are      These sand lobes are interpreted to be part of a new
 overlain by small channelized conglomerates, com-          lowstand systems tract (Posamentier & Vail, 1988)
 posed of resedimented neritic carbonates. The top          that formed during the late Paleocene, probably at
 of the section is composed of thin-bedded turbidites       approximately the 58.3 Ma sea-level fall (Haq et al.,
 and slump deposits.                                        1988). The overlying interval of thin-bedded tur-
                                                            bidites and shales indicates a drastic decrease in sili-
                                                            cic1astic sediment supply, most probably owing to a
 Interpretation
                                                            rapid sea-level rise. These deposits are interpreted
In the Tempisque Trough, four depositional second-          to be part of the late lowstand systems tract as well
order sequences have been identified. However, se-          as of the transgressive and highstand systems tract.
quences vary in size, geometry, and location.               In the northern basin area, the sequence boundary
  Seque/lce I is mainly exposed in the southern             between sequence 1 and sequence 2 is indicated by
basin area (section D) and consists entirely of un-         the occurrence of very coarse-grained, volcanic1astic
channelized lobe deposits. As in section B, the             channel-fill deposits. Biostratigraphic data suggest
sequence boundary is indicated by the abrupt onset          that deposition of these channelized conglomerates
of sand lobes during the latest Maastrichtian at            occurred at approximately the late Paleocene
approximately the 68 Ma sea-level fall (Haq et al.,         58.3 Ma sea-level fall (Haq et al., 1988). This channel
1988). These sand lobes arc interpreted to be part of       complex is interpreted to be part of the lowstand
the lowstand systems tract. The large thickness of          wedge (Posamentier & Vail, 1988). The overlying
these lobe sediments indicates that very high sedi-          late Paleocene to early Eocene fine-grained cal-
mentation rates occurred during early Paleocene             careous mudstones and ca1carenites are interpreted
Fore-arc deep-water response                                        281

to have been fonned during the subsequent sea-level     siliceous mudstones and limestones of early to late
rise and to be part of the transgressive/highstand      Campanian age (Baumgartner et al., 1984), which
systems tract.                                          unconfonnably overlie the basaltic basement or its
   The boundary between sequence 2 and sequel/ce 3      cover of basaltic breccias.
is indicated by the onset of lobe deposits, both in        From late Maastrichtian to late Paleocene times a
section C and section D. In section C the onset of      coarse-grained submarine fan complex developed.
lobe deposits most probably occurred during early       Foraminiferal biostratigraphy indicates that the
mid-Eocene times (A. Astorga, personal communi-         transition from calcareous limestones and mudstones
cation, 1990) and may be related to the 49.5 Ma sea-    to siliciclastic deposition occurred during the ter-
level fall (Haq et al., 1988). In section D, no bio-    minal late Maastrichtian (NC22/NC23; Schmidt-
stratigraphic control is available. The lobe deposits   Effing, 1979).
are interpreted to be part of the early lowstand           A basal lobe complex consists of unchannelized,
systems tract, whereas the overlying thin-bedded        medium to thick-bedded sandstones, which are rich
turbidites most probably were deposited during the      in resedimented neritic fossils. These lobe deposits
late lowstand systems tract and the transgressive and   are overlain by channelized, chaotic mass flow
highstand systems tract. In section C, lower sedi-      deposits, including large, well-rounded andesitic
mentation rates can be deduced from minor sediment      boulders up to 3 m across and huge blocks of both
thicknesses and the finer grained nature of the         Upper Cretaceous and Paleocene platfonn carbon-
deposits.                                               ates (Calvo, 1987). Up section, a further lobe com-
   Sequence 4 is only recorded in section D. The        plex consists of channelized, massive to thick-bedded
vertical facies arrangement of these upper Eocene       sandstones. This lobe system initially fonned during
sediments indicates a typical lowstand systems tract.   the late Paleocene (P4; Lehfeld, 1989) and is overlain
The sequence boundary is marked by the onset of a       by coarse-grained channel-fill deposits, which are
new lobe complex, indicating sedimentation during       very rich in resedimented neritic fossils and carbon-
the early lowstand systems tract. Overlying chan-       ate debris. These conglomerates and chaotic mass
nelized conglomerates are interpreted to have been      flow deposits are overlain by thin-bedded turbidites,
deposited during the early stage of lowstand-wedge      which pass upwards into siliceous limestones and
fonnation. The slump and debris apron on top of the     mudstones of late Paleocene to early Eocene age
sequence are interpreted to be a part of the later      (Schmidt-Effing, 1979; Baumgartner et al., 1984;
stage of lowstand wedge fonnation, when sedi-           Lehfeld, 1989).
mentation within channels had stopped, and sedi-
ment failure in the slope was caused by rapid
                                                        illterpretation
progradation of lowstand deltas (Posamentier &
Vail,1988).                                             Sequence 1 is built up by a typical lowstand systems
                                                        tract with basinal lobe deposits and an overlying
                                                        lowstand wedge. The sequence boundary is marked
    DEPOSITIONAL SEQUENCES IN                           by the onset of sand lobes, indicating sedimentation
       TRENCH SLOPE BASINS                              during the early lowstand systems tract. The tran-
                                                        sition from mainly biogenic sedimentation to silici-
Several small slope basms developed on the seaward      clastic deposition occurred dunng the terminal
margin of the outer structural high. Deposits of        Maastrichtian at approximately the 68 Ma sea-level
these trench slope basins are documented in the area    fall (Haq et al., 1988). The overlying channelized
of Samara, Cabo Blanco, and Quepos (Figs I, 4).         conglomerates and chaotic mass flow deposits are
Facies associations within the sedimentary record       interpreted to be part of the lowstand wedge. Sec-
vary considerably and strongly depended on local        tions of the transgressive and hlghstand systems
conditions, such as size and location of submarine      tract do not occur.
canyons as well as differential uplift/subsidence of       The boundary between sequence \ and seqllence 2
the outer fore-arc area.                                is indicated by the deposition of a new lobe system.
                                                        The onset of these lobe deposits occurred during the
                                                        late Paleocene at approximately the 5S.3 Ma sea-
Section E
                                                        level fall (Haq et al .• \988) and reveals sedimentation
The oldest sediments of the Samara area consist of      in an early stage of the lowstand systems tract. The
~
                                                                                                          G

                                                                 F
                                                                                                                            -
                                                                                                                            E2J
                                                                                                                                       carbonate
                                                                                                                                       megabrecclas

                                                                                                                                       channellzed sand-
                                                                                                                                       stones and conglomerates

                       E
                                             OJOJ
                                               C
                                               OJ
                                             -!:j
                                                      ).
                                                    ~~~~~~'
                                                           '.
                                                            "'   ..... . .
                                                                 .....
                                                    ~~ •. "':J~.:: .. ::-;~
                                                                                                                            o           sand lobes

                                                                              ""                              SB 39,5   7

                                                                                    ~

                                                                                        "
                                                                                                                            o          thin - bedded
                                                                                                                                       turbld,tes
                                                                                                                                                                      ~

                                                                                                                                                                      ~
        >.OJ                                                                                                  SB 49,5")                                               ~r::.
       -c
       .... OJ
       01,)
       Wo
          W
                                                                                                                            o           calcareous mudstones
                                                                                                                                        and thin - bedded
                                                                                                                                        turb,d,tes
                                                                                                                                                                      :::
                                                                                                                                                                      :::
                                                                                                                                                                      §
                                                                                                                                                                      r::...
                                              "                                                                                         sllrceous limestones          ::z:
                 I~~}~\;m~m:~~i
                                                                                                                            ~
                                                                                                                            EEa         and mudstones
           OJ
           C
          OJ                                                                                                                                                          s.~
          I,)
        OJo
       -OJ
                                                                                             OIJ")
                                                                                             0..0..
                                                                                                                                        silICeous and
                                                                                                                                                                      [
       0_                                                                                    (1)_
                                                                                                                            rmHill]
                                                                                        -- -"
       -'0
          0..                                                                                00..                                       calcareous mudstones
                                  -")   --                                    -   -")                         SB 58,3 ")
                                                                                              :>,..J
                                                                                             L:~,
                                                                                             LE       I

                                                                                                                                        pelaglc limestones
                                              OJ

                                             11                                                                                         sequence boundary

                                                                                   SB 68")
                                                    ~i....~.i....i..l.i:

Fig. 4. Basin-wide correlation of depositional scquence~ in deep-water ~ediments of the outer fore-arc area of Costa Rica. Location of columns arc shown in Fig. 1.
Age determination from Azt!ma et al. (1979), Schmldt-Effing (1979), Baumgartner et al. (1984), and Lchfeld (1989).
Fore-arc deep-water response                                        283

overlying channel-fill deposits are interpreted to         taceous biogenic sediments. This transition most
have been formed during the lowstand wedge. The            probably occurred during the clearly defined 68 Ma
increase in reef and shelf-derived shallow-water           fall of sea level (Haq et al_, 1988), when coarser
fossils indicates a relative rise of sea level, which      terrigenous sediments were also funnelled into
caused a re-establishment of shallow-water pro-            smaller canyons of the trench slope, leading to the
duction during the late lowstand wedge. The fine-          formation of intercalated siliciclastic turbidites and
grained siliceous limestones and mudstones on top          hemipelagic mudstones. The overlying fine-grained
of the sequence reflect a rapid rise of sea level with a   hemipelagic mudstones are interpreted to have been
drastic decrease in sediment supply and are in-            formed during the subsequent sea-level rise and
terpreted to have been formed during the trans-            be part of the transgressive and highstand systems
gressive and highstand systems tracts_                     tracts.
                                                              The boundary between sequence 1 and sequence 2
                                                           is marked by the new onset of siliciclastic turbidites.
Section F
                                                           which probably occurred during late Paleocene
In the area of Cabo Blanco, thick siliciclastic tur-       times. The occurrence of these siliciclastic turbidites
bidite successions are absent and biostratigraphic         is related to lowstand deposition, whereas the over-
control is very poor. Instead, fine-grained calcareous     lying siliceous limestones and mudstones are inter-
mudstones and thin-bedded turbidites overlie the           preted as highstand deposits.
oceanic basement and Upper Cretaceous biogenic                The boundary between sequence 2 and sequence 3
rocks (Lundberg, 1982; Baumgartner et al., 1984;           is indicated by the occurrence of channelized cal-
Astorga, 1987).                                            carenites and conglomerates. Lowstand deposition
   The basinal section consists of Paleocene thin to       IS marked by the appearance of significant sand- and
medium-bedded siliciclastic turbidites, which are          pebble-sized volcanic material as well as the re-
intercalated with fine-grained siliceous and cal-          sedimentation of large amounts of larger Foramin-
careous mudstones (Lundberg, 1982; Baumgartner             ifera, which indicate erosion of carbonate shoals
et al., 1984). Turbidites show a thickening up-            during an initial fall of sea level (Sarg. 1988).
ward trend and are overlain by siliceous and cal-
careous mudstones. These mudstones are thin to
                                                           Section G
medium-bedded and individual beds show subtle
grading of foraminiferan tests, representing locally       The Quepos section has unusual facies. Specific
derived hemipelagic material that was redeposited          characteristics of this basin-fill system show close
by minor currents. Up section, a new interval of           similarities with wrench-fault basins. described from
interbedded mudstones and sandstones occurs. This          the Californian borderlands (Howell et al., 1980)
interval is characterized by a progradational and          and the eastern Betics (Montenat et al .• 1987). The
retrogradational cycle. The uppermost section con-         depositional regime was characterized by repeated
sists of siliceous limestones and mud stones of            fault activity, resulting in deepening of the basin.
probably late Paleocene to middle Eocene age               followed by rapid erosion of borderlands and sedi-
(Baumgartner et al., 1984). On top of the section,         mentation of very coarse clastic sediments on a
minor channelized calcarenites and conglomerates           narrow coastal fringe of fan-deltas leading into sub-
occur. These channel-flll deposits are middle Eocene       marine fans. Facies development is divided into
in age (Baumgartner et al., 1984) and consist of           coarse-grained megabreccias and small conglom-
resedimented neritic carbonates, isolated larger           eratic cones along the basin margin and a finer
Foraminifera and volcanic rock fragments.                  grained turbiditic facies along the basin aXIs.
                                                              Basal slope strata overlie probably accreted alka-
                                                           line basalts (Sick. 1989) and basaltIC sedimentary
JlIlerpretation
                                                           breccias. The basaltic breccias are of early Paleocene
The absence of thick sand-rich turbidites suggests         age (Plb/Plc; Schmidt-Effing. 1979) and contain
the occurrence of smaller submarine canyons down-          radiolarites and Late Cretaceous limestones (Azema
slope of the outer structural high that did not receive    et al .• 1979). which point to the existence of an older
a direct supply of terrigenous detritus.                   basement In this area (sce. for comparison. the
   The sequence J boundary is indicated by the onset       paper by Seyfried et al.). Pelagic carbonates. which
of Paleocene siliciclastic turbidites over Upper Cre-      subsequently formed (P2/3A; cf. Azcma et al. 1979;
284                                      J. Willsemallll alld H. Seyfried

Schmidt-Effing, 1979), indicate a very low sediment        mudstones. Similar to sequence 1 of section E, this
input up to the early late Paleocene.                      deposition may indicate a lowstand during early
   The overlying strata consist of siliceous mudstones     Paleocene times. when sman-sized submarine
and thin-bedded sands tones and are very similar to        canyons received a larger amount of terrigenous
the deposits of the Cabo Blanco area. The mudstones        material.
are rich in Radiolaria and planktonic Foraminifera             The boundary between sequence 1 and sequellce 2
with an up-section increase of calcareous oozes.           is indicated by deposition of megabreccias. These
Sandstones are mainly composed of volcaniclastic           megabreccias initiany formed during the late Paleo-
rock fragments. Some sandstones also contain               cene at approximately the 58.3 Ma sea-level fall
basaltic and radiolaritic rock fragments, which were       (Haq et aI., 1988). As deposition of breccias con-
recycled from the Nicoya Complex.                          tinued until early Eocene times it is suggested that
   Biostratigraphic data indicate an early Paleocene       tectonic control on deposition owing to uplift and
to early Eocene age (P2/P3B - P7/P8; Azema et al .•        tilting of the outer fore-arc was the dominant con-
1979; Schmidt-Effing. 1979). These dominantly fine-        trolling factor. However. well-rounded clasts of ner-
grained deposits are associated with chaotic mass          itic carbonates as wen as basalts and radiolarites
flow deposits of late Paleocene to early Eocene age         indicate erosion of neritic environments. which
(Schmidt-Effing. 1979). The chaotic mass flow de-           probably occurred because of the 58.3 Ma sea-level
posits consist of megabreccias with huge blocks of         fan (Haq et al .• 1988) and interfered with tectonism
siliceous limestones. up to lOOm across. Resedi-            in this area.
mented neritic carbonates are rich in larger Fora-             The boundary between sequence 2 and sequellce 3
minifera of late Paleocene age and make only a              is marked by the onset of wen-organized, braided
minor contribution to the components. The matrix is         channel systems. Wen-rounded clasts of conglom-
composed of siliceous and calcareous mudstones.             erates and high contents of larger Foraminifera may
similar to the underlying deposits. Biostratigraphic        indicate a major eustatic control on deposition.
ages fan within planktonic foraminiferal biozone               The internal facies construction of late Eocene
P4/5 (Schmidt-Effing, 1979).                                deposits indicates a clear eustatic control on depo-
    Rapid uplift in middle Eocene times resulted in         sition, resulting in the formation of typicallowstand
 the formation of sman, very coarse-grained braided         systems tracts. The boundary between sequence 3
 channel systems, which most probably were related          and sequellce 4 is marked by the deposition of large-
 to fan-deltas. The clast composition is still dominated    scale channel-fill deposits and probably occurred
 by recycled material from the Nicoya Complex                because of the late Eocene 39.5 Ma sea-level fall
 (basalts, cherts, and pelagic carbonates), but also         (Haq et al.. 1988). The vertical development of
 includes coarse volcanic detritus, derived from the         coarse-grained channelized conglomerates and sand-
 calc-alkaline arc. Age determination from resedi-           rich. channel overbank complexes are interpreted to
 mented larger Foraminifera indicates deposition in          be part of a lowstand wedge. Lobe deposits. which
 early middle Eocene times (Azema et al., 1979). The         overlie the channel overbank complex. indicate a
 coarse-grained middle Eocene deposits are overlain          third-order sequence boundary and are interpreted
 by upper Eocene sand-rich turbidites. These upper           to have been formed during a rapid fan of sea level
  Eocene deposits start with coarse-grained. chan-           and are therefore interpreted to be part of a new
 nelized conglomerates. which pass up-section into           lowstand systems tract. The final interval of thin to
 sand-rich channel-fin deposits. These sand-rich             medium-bedded turbidites. which overlies the lobe
 channel fills are associated with overbank deposits         deposits. formed during the subsequent lowstand-
 and are overlain by a thick overbank wedge. The             wedge stage.
 uppermost section is composed of lobe deposits.
 which consist of medium to thick-bedded turbidites.
 rich in larger Foraminifera. Sand lobes are overlain
 by an interval of thin to medium-bedded turbidites.           EFFECTS OF TECTONIC UPLIFTI
                                                              SUBSIDENCE, VOLCANIC SUPPLY,
                                                              AND EUSTATIC FLUCTUATIONS ON
Illferpretatioll
                                                                       DEPOSITION
Sequellce I is not completely exposed and consists of
intercalated thin-bedded turbidites and hemipelagic          At the end of the late Maastrichtian, sedimentation
Fore-arc deep-water respollse                                             285

relative lowstand - moderate uplift - very high sediment input

            sand lobes

.,          braid delta

 D          tal us

Fig.5. Very high sedimentation rates and a relative lowstand of sea-level caused the formation of thIck lobe systems in the
southern Tempisque Trough. This preferred development of sand-lobe systems from early Paleoccne to Eoccne times was
most probably favoured by a small, elongate basin shape In this area, and very high sedimentation rates during this tIme
may have outpaeed the rate of sea-level rise dunng early Palcoccne times.

changed markedly, and sand-rich turbidite systems              event affected the northern parts of the fore-arc
formed both in the inner and outer fore-arc troughs            area. This event led to the destruction of Upper
owing to the 68 Ma sea-level fall (Haq et al., 1988).          Cretaceous platform carbonates, which were de-
In the southern Tempisque Trough, extensive sand               posited in both the northern Tempisque Trough and
lobe systems formed. For the first time in the evol-           the southeastern Nicaragua Trough. The subsequent
ution of the island arc, there must have been a                58.3Ma sea-level fall (Haq et al., 1988) interfered
neritic system, which was able to store large amounts          with an episode of major tectonic/volcanic activity.
of sand. Lobe sedimentation persisted during the               As a result, a coarse-grained turbidite system de-
entire Paleocene with an up-section increase in grain          veloped (Fig. 6). The concomitant uplift and tilting
size. The overall thickening and coarsening upward             of the outer fore-arc area (see paper by Seyfried
cycles within these Paleocene lobe systems indicate            et al.) caused back-cutting of submarine canyons
that a very high sediment influx, delivered from the           that deeply incised across the outer arc and headed
rising calc-alkaline island arc, was able to compen-           on to the narrow shelf of the calc-alkaline arc. At
sate the effects of sea-level rises during Paleocene           the mouth of these canyons, very coarse-grained
times (Fig. 5). In addition, the preferred develop-            submarine fans formed in the outer fore-arc area
ment of sand lobe systems may have been favoured               (Fig. 7). Where smaller submarine canyons only
by an elongate basin shape which funnelled axial               headed on to the slope of the outer arc, locally
flows in this area.                                            derived mass flow deposits and thin-bedded tur-
   In the early Paleocene, a major tectonic/volcanic           bidites prevailed.
286                                       1. Willsemalll/ alld H. Seyfried

relative lowstand - strong uplift - high sediment input - volcanic activity

          coarse to very coarse
          channel- till deposits
           scarp breccias,
           boulder avalanches

Fig. 6. Strong uplift, high sedimentation rates. and a relative lowstand of sea level caused the formation of channelized
lobes and very coarse-grained channel-fill deposits in the inner fore-arc area of southwest Nicaragua and northwest Costa
Rica during the late Paleocene and late Eocene.

   The subsequent sea-level rise in the latest Pal eo-         creased again and ash-rich sediments were deposited
cene and the early Eocene is mainly documented by              in the inner fore-arc basins. Common resedimented
retrogradational cycles. In addition, the calc-alkaline        larger Foraminifera indicate the persistence of neritic
arc was largely inactive throughout late Pa1eocene to          carbonate systems upon the shelf. Small to moderate
early Eocene times and contributed only minor                  drops of sea level are documented by the formation
volcaniclastic sediments to the basins. In the inner           of minor siliciclastic slope fans, rich in resedimented
fore-arc troughs, thin-bedded turbidites formed                larger Foraminifera (Fig. 10).
where siliciclastic neritic systems persisted, whereas            During the early late Eocene, volcanic/tectonic
siliceous mudstones and limestones developed when              activity intensified and very coarse-grained sub-
carbonate ramps were establised (Fig. 8). The uplift           marine fan systems formed. Fragmentation of the
of the outer structural high during the late Paleocene         basin margin caused the destruction and redeposition
closed the S
Fore-arc deep-water response                                            287

relative lowstand - strong uplift - high sediment input - volcanic activity

            coarse -grained                   ~
             channel-lobe systems             ~           carbonate ramp

             siliceous limestones
D            thin - bedded turbidites

Fig. 7. A model for submarinc fan sedimentation in thc outer forc-arc during thc early Paleoccne and carlicst late
Paleocene. Strong uplift of the fore-arc area. high scdlmentation rates. and a rclative lowstand-of sea level caused the
formation of sand lobes and vcry coarse-grained channel-fill deposits in the outcr fore-arc area. wherc submarinc canyons
deeply incised into the outer arc and headcd on to the narrow shelf of the volcamc arc. Wherc smallcr submarine canyons
only headed on to the slope of the outcr arc, locally dcrived mass flow dcposits and thin-bcdded turbiditcs prcvalled.

strongly related to the morphotectonic evolution of            determination of the larger scale stratal pattern or
the island-arc system. Each depositional sequence              potential condensed sections are eroded. Third-
reflects the complex interaction between global sea-           order depositional sequences could be determined
level fluctuations, sediment supply, and tectonic              only when high rates of sedimentation and sub-
activity. Sediment supply and tectonic activity over-          sidence occurred.
printed the eustatic effects and enhanced or lessened             Though lowstands of sea level favour the develop-
them. If large supplies of c1astics or uplift overcame         ment of turbidite or fan systems, the amount and
the eustatic effects, deep marine sands were also              type of sediments will significantly influence the type
deposited during highstand of sea level, whereas               and timing of turbidite events. Very coarse channel-
under conditions of low sediment input, thin-bedded            lobe systems will only form in mature island-arc
turbidites were deposited even during lowstands of             systems when simultaneous tectonic uplift and high
sea level.                                                     volcanic activity occur in addition to sea-level
   The internal facies architecture of depositional            lowering. During periods of tectonic and volcanic
sequences can be reduced strongly, and recognition             quiescense, fine-grained, thin-bedded turbidite
of transgressive and highstand systems tracts may be           systems prevail even during lowstands of sea level.
uncertain, where outcrop data do not allow                        Remarkable changes in sedimentation occurred
288                                         1. Willsemallll alld H. Seyfried

relative highstand - tectonic quiescence - low sediment input

             siliceous mud                   1c:C(;'~;~   barrier sands

             carbonate oozes                 ~            estuary

             carbonate ramp
             (forams + red algae)
                                             ~            flood plain

Fig. 8. A relative highstand of sea level, tectonic quiesccnse, and low sedimentation rates during late Paleocene to early
Eocene times caused the formation of siliceous mudstones and limestones in the inner fore-arc basins, where carbonate
ramps became established. Where siliciclastic ncritic systems persisted, thin-bedded turbidites formed in the adjacent basin
area~.

when tectonic/volcanic pulses and second-order                   basins, resulting in increased carbonate contents in
cycles of eustatic fluctuation interfered. Therefore,            the basinal sediments. Conversely, during sea-level
uplift and subsidence of sediment-source areas acted             lowstands, subaerial exposure of platforms caused
as major controls on deposition of basinal cycles.               meteoric cementation and karstification and, as a
Alternate immersion and exposure of the carbonate                result, only a little mud- and sand-sized carbonate
shelf that supplied the shallow-water carbonate sedi-            shelf detritus reached the deep sea. In ramp settings
ments into the fore-arc basins caused long-term                  a sea-level fall exposes only the upper parts of
hybrid siliciclastic-carbonate basin cycles. The                 carbonate ramp systems and production of shallow-
abruptness of Iithologic transitions between carbon-             water carbonate sedimerit continues in the outer
ate breccias and siliciclastic deposits suggests that            ramp environment (Shanmugam & Moiola, 1985;
resedimentation of carbonate breccias mainly oc-                 Sarg, 1988; Dolan, 1989). Such moderate or initial
curred owing to tectonic/volcanic events. Siliciclastic          lowstands are documented by grainflow deposits and
sedimentation indicates that most detritus reached               turbidites, rich in larger Foraminifera.
the deep sea during relative sea-level lowstands,                  The most striking feature is the almost simultaneity
when the sediment bypassed subaerially exposed                   of tectonic/volcanic activity and second-order cycles
shelves. Basinal siliciclastic sedimentation decreases           of eustatic sea-level fluctuations during Late Cre-
when most of the detritus was trapped in inner shelf             taceous, Paleocene, and mid-late Eocene times.
environments. During relative highstands, mud- and                  Many causes have been proposed for changes In
sand-sized carbonate sediment was shed off neritic               global sea levels, by changing either the total volume
carbonate systems and transported into the fore-arc              of sea water or the total volume of the oceanic
Fore-arc deep-water respOl/se                                              289

          relative highstand - tectonic quiescence - low sediment input

                                                                      carbonate ramp
                       siliceous muds
                                                                      (forams + red algae)

         o             carbonate oozes

         Fig_ 9_ A relative highstand of sea level. tectonic quiescensc. and low sedImentation rates cau!>Cd the formation of ~ihceou~
         mudstones and limeMones in the outer fore-arc basins dunng late Paleocene to middle Eocene tImes. Carbonate ramp~.
         which concomitantly developed on the outer structural high. periodically delivered neritic carbonate matenal to the lower
         slope.

         basins (Pitman, 1979; Howell & von Huene, 1980;                  with global orogemc phases which in turn coincide
         Miall, 1984). C10etingh (1988) proposed a te~tonic               with crusta I movements in Central America. These
         mechanism for apparent sea-level fluctuations with a             episodes of major global tectonic/volcanic activity
         maximum magnitude of a few hundred metres. Ac-                   occurred at 80-75, 63, 55-53, and 42-38Ma and
         cording to this model, tectonically induced vertical             agree with major tectonic and volcanic pulses in
         motions of the lithosphere may cause third-order                 Central America (see paper by Seyfried et al.).
         cycles. Larger fluctuafions in apparent sea level could          This corroborates that global tectonic processes play
         be related to major re organizations of lithospheric             an important role m generating major sea-level
         stress fields owing to rifting and fragmentation of              fluctuations and hence in the formation of de-
         plates, dynamic changes at convergent plate bound-               positional sequences.
         aries, or collision processes. Schwan (1980) compiled
         data showing that world-wide orogenic activity in
         the Late Jurassic to Recent, mcluding the develop-               ACKNOWLEDGEMENTS
         ment of unconformities and compressional struc-
         tures, metamorphic and plutonic episodes, could be               This study was supported by the Deutsche For-
         correlated with major readjustments in the world                 schungsgemeinschaft (DFG; Se 490/1-1, Se 49{)/I-
         plate patterns.                                                  3). The field trips were sponsored by grants of the
            Late Cretaceous to late Eocene drift rates calcu-             Deutscher        Akademischer      Austauschdienst
         lated from the North Atlantic Ocean correlate closely            (DAAD). The research in Costa Rica was further

~-----
relative highstand - slight uplift - low sediment input - volcanic activity
                                                                                            &~ /~~
                                                                                             \~:~-'   I \ I

                                                          tuffaceous muds                                     nearshore sands

                                              D           minor slope fans
                                                                                          ~                   barrier sands

                                              LJ..
                                                           mixed carbonate-
                                                           clastic system                 D                   flood plain

Fig. 10. A relative highstand of sea level, slight uplift, and low sedimentation rates dunng the middle Eocene led to the
formation of tuffaeeous mudstones as well as thin-bedded turbidites. Smaller slope fans developed as a result of a minor sea
level lowstand or a local increase in volcaniclastic supply.

relative lowstand (rising sea level) - tectonic quiescence - high sediment input

•
f7'l
LJ
           lowstand delta

           channel- overbank systems, slumps, chaotic mass-flow deposits, baSin-plain turbldltes

Fig. H. A relative nse of the sea level in late Eocene times changed the coarse-graincd sedimentation shown in Fig. 6 and
channcl ovcrbank systcms dcvcloped. Overbank deposits and overbank wedgcs arc rieh in organic matcrial and must have
been related to lowstand deltas.
Fore-arc deep-water respo1lse                                               291

supported by the close cooperation with the national                   Costa Rica): ihre "erbreitung. Fazies und geologische
oil company of Costa Rica (RECOPE) and the                             Geschicllle mll besonderer Berucksichtigung der Radio-
Department of Geology. University of Costa Rica.                      larite. Unpublished dissertation. University of Marburg.
                                                                      394 pp.
Transport facilities and field support were further                HAQ. B.U .• HARDENBOl. J. & VAll. P.R. (1988) Mesozoic
provided by the Instituto Nicaragliense de la Mineria                 and Cenozoic chronostratigraphy and eustatic cycles. In:
(lNMINE). We would like to thank David Mac-                           Sea-lel'el Changes: all Integrated Approach (Eds Wilgus.
donald (British Antarctic Survey) for his helpful                     e.K .. Hastings. B.S .. Kendall. e.G. St. e.. Posamentier.
                                                                       H.W .• Ross. e.A. & van Wagoner. J.). Soc. Econ.
comments and review of this paper.                                    Paleontol. Mineral.. Spec. Publ. 42. 71-108.
                                                                   HOfHERR. G. (1983) Geologia de las ealizas de Sapoa.
                                                                       In! Final Proy. 05. Unidad Geol. COMNOMET
                                                                      (unpublished). Instituto Nicaraguense de la Mineria
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