10 Architecture and facies associations of Plio-Pleistocene trench-slope deposits, Burlca Peninsula, Central Amerlca R. Schlegelt, U. Wortmann2, H. Krawinkel\J. Krawinkel1 andJ. Winsemann3

'Institut für Geologie und Paläontologie, Universität Stuttgan , Herdweg SI , 701 74 Stuttgan, Germany

'Institut für Angewandte Geologie und Mineralogie, Technische Universität München, Lichtenhergstr. 4, 85748 Garching, Germany

'Geologisch-Paläontologisches Institut und Museum, Universität Hamburg, Bundesstrasse 55 , 201 46 Hamburg, Germany

lntroduction The Burica Peninsula (Costa Rica/W-Panama) belongs to the forearc area of the Central American arc-trench system which extends along southern Mexico and the Central American isthmus. The southern part of the Central American Iandbridge is the product of an island arc (Seyfried et al. 1991) and was initially formed in the early Late Cretaceous (Winsemann 1993). Detailed descriptions of the forearc basin-fill systems have recently been published by Kolb and Schmidt (1991), Schmidt and Seyfried 0991), Winsemann and Seyfried 0991), Amann 0993), Eynatten et al. 0992, 1993) and Winsemann (1992).

The plate tectonic configuration is outlined in Fig. 10.1. Trench-parallel, left-lateral strike-slip movements are considered to be the dominant mechanism in the development of the depositional environments of the study area. The 'Ballena-Celmira Fault Zone' (Corrigan et al. 1990), or the 'Longitudinal Fault' (Seyfried et al. 1991), probably represents the most prominent structural feature north of the Burica Peninsula. Parallel to this active fault , shear movements and related conjugate sets of strike-slip faults have affected the sedimentary sequence. This is documented in lineation geometries obtained from satellite imagery and aerial photographs, in addition to outcrop data analysis. Large strike-slip faults are the main controlling factor of basin geometries. Steeply dipping reverse faults within the 'Medial Fault Zone' (Corrigan et al. 1990) represent part of a north-south striking positive flower -structure.

The basement rocks (Nicoya Complex) consist

mainly of T-Morb, N-Morb, IAT and radiolarites, with ages ranging from early Cretaceous to mid-Eocene times (Seyfried et al. 1991). They are overlain by bioclastic and biomicritic Iimestones of late Palaeocene to Eocene age which are poorly exposed (Corrigan et al. 1990) An at least 3.5 km thick sequence of Pliocene to Pleistocene volcaniclastic marine sediments of the Charco Azul Formation unconformably overlie the basement. According to Corrigan et al. 0990), the Charco Azul Formation can be divided into three members and consists of a basal unit of shallow marine deposits (Peiiita Member), a middle unit of deep-water deposits (Burica Member) and an upper unit (Armuelles Member) recording the rapid shoaling from deep to shallow water condition. In this chapter detailed sections of the Burica Member are presented, giving an excellent insight into the architecture of a continuous section of trench slope deposits.

Facies description of the Burica Member The turbidite systems of the Burica Member crop out along the wave-cut platform of the Burica Peninsula between Punta Gorda and Quebrada Corotu (Fig. 10.2). The thickness of the Burica Member is c. 3500 m. Two major facies associations can be distinguished:

1. A slope-basin association in the lower part of the section. Facies associations comprise coarse­grained channel-fill deposits, sand-rich channel­fill deposits with associated overbanks and tabular lobe deposits.

2. A slope association in the upper part of the section. The slope association consists of mud­dominated sand-mud couplets. In the lower part of this slope association intercalations of slumps and internally dislocated, brecciated and balled strata frequently occur, whereas upsection intercalations of disorganized gravels, muddy gravels, gravelly muds and pebbly sandstones prevail. Facies descriptions of these turbidites are based on the classification of Pickering et al. 0986) (Table 10.1).

Atlas of Deep Water Environments: Architectural style in turbidite systems. Edited by K.T. Pickering, RN. Hiscott, N.H. Kenyon, F. Ricci Lucchi and R.D.A. Smith. Published in 1995 by Chapman & Hall, London. ISBN 0 412 56110 7.

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Table 10.1. Facies associations of the Burica Member

Slope association

Slope-basin association

Interpretation

Slope deposits

Lobe deposits

Overbank deposits

Channel-fill deposits

Unchannelized mass

Facies

A2.2, A2.3, A2.7 Al.l-A1.3 F2.1 D2.2 C2.2-C2.4 D2.2 C2.1-C2.4 B2.2 C2.1-C2.3 C2.1-C2.3 B2.1-B2.2 A2.2-A2.4, A2.7, A2.8 Al.l, A1.4

flow deposits A1.3

...

Columns

Fig. 10.1. Plate tectonic configuration (modified after MacKay and Moore 1990). The Cocos Plate is currently being subducted beneath Costa Rica at 9 cm/yr (Burbach et al. 1984). The plate boundary between Cocos Plate and Nazca Plate is formed by the Panama Fracture Zone which initially evolved in the late Miocene (Lowrie et al. 1979). The aseismic Cocos Ridge reached the subduction zone one million years . ago and is recently being subducted beneath the Burica Peninsula (Lonsdale and Klitgord 1978). A major strike-slip fault system (partly mentioned as the 'Medial Fault Zone' by Corrigan et al. 1990) structurally dominates the study area (Burica Peninsula).

referring to Fig. 10.5 Sequences

10-15 Bu-III

6-9 Bu-II

3-5

3-5 Bu-I

1-2

1-2

•

•

Fig. 10.2. Location of sections. Bars indicate adjoining outcrops along the wave-cut platform.

Fig. 10.3. (Right) Compilation of the measured data of the complete Burica Member (modified after Wortmann 1992). The main facies associations and third-order depositional sequences (Bu-I-Bu-IV) are indicated along the left side of the section. Sequence boundaries are indicated by major changes of facies associations or abrupt increases of the sand-mud ratio. The compositional variation of the sections are indicated along the right side of the section. Each sequence displays characteristic trends of sandstone composition (Schlegel 1992). Sequence Bu-I is characterized by fragments of basalts, metamorphic limestones, quartz and a minor portion of radiolarites, which indicates reworking of the underlying basement rocks. The Bu-II sequence is characterized by the first occurrence of andesitic clasts, derived from the volcanic arc. Simultaneously, an increase in feldspar and a decrease in quartz can be observed. The Bu-III and Bu-IV sequences show a similar clast composition and are characterized by a gradual increase of andesitic fragments.

Four third-order depositional sequences can be determined (Fig. 10.3). Sequence boundaries are indicated by major changes of facies associations. In the following the depositional sequences are referred to as Bu-I, Bu-II, Bu-III and Bu-IV.

Bu-1 sequence The depositional sequence Bu-l forms the oldest part of the section and crops out at the Isla Burica as weil as at the southern platform of the Burica Peninsula. The basal sequence is characterized by sheet-like muddy debris flow deposits and very coarse-grained channel-fill deposits. Upsection, the coarse-grained channel-fill deposits are overlain by sand-dominated channel-fill deposits and associated overbank complexes. The debris-flow deposits range from 1 to 10m in thickness and alternate with 1-15 m thick units of thin-bedded sand-mud couplets (C2.3) and thick­bedded mud-dominated sand-mud couplets (C2.4). The debris flow deposits consist of matrix-supported (> 60%) gravelly muds. The components consist mainly of mud clasts with an average diameter of 5- 20 cm, maximum sizes are up to 1m (A1.3). Commonly,

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disorganized pebbly sandstones (0.9-1.5m in thickness) occur right below the base of the mud­supported debris-flow deposits.

The overlying chaimel-fill system is characterized by thick-bedded gravels, which commonly pass into disorganized or graded-stratified pebbly-sandstones and sandstones (A2.2, A1.4, A2.8 and B2.1). Bed thicknesses range from 1 to 6 m. The channels are up to 100 m wide with an average depth of 10m.

The channel-fill deposits of the channel-overbank complex on top of the sequence are built by braided systems. Average dimensions of channels range from 10 to 30m in width and 1-5m in depth. The channel­fill deposits consist mainly of amalgamated thick­bedded sand bodies of parallel-stratified sandstones (B2.1). The associated overbank facies consist of thin­bedded sand-mud couplets with well-developed Tbcde divisions. Sametim es Ta divisions can be observed. Upsection the occurrence of channel-fill deposits decreases.

Bu-11 sequence

This depositional sequence crops out along the south­eastern part of the Peninsula Burica between Punta Burica and Quebrada del Medio. The basal lobe complexes consist of sand-mud couplets displaying an overall thickening-upward and coarsening-upward trend. The internal organization is characterized by

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both fining and thickening-upward trends. Facies types range from thin-bedded turbidites (C2.3) and medium­to-thick-bedded turbidites (C2.2/ C2.1) commonly with Tbcd divisions.

Upsection, the lobe deposits are overlain by mud­dominated sand- mud couplets (C2.4) and thick silt-mud couplets (D2.2). This part of the section shows an overall fining-upward trend. Intercalations of small-scale slides represent typical features. This facies association is interpreted to represent slope deposits. The ichnofabric within the basal part of the sequence comprise the ichnospecies 7balassinoides, Helminthopsis and Teichichnus and is mainly characterized by vagile and stationary mid-tier deposit­feeder structures. A gradual shift of the ichnofabric occurs within the upper part of the sequence. Chondrites, Gyrophy lites, Helminthopsis, Teichichnus and Nereites (?) occupy the deep Ievels of a bioturbated sequence and comprise the deepest tiers.

Bu-111 sequence

This depositional sequence crops out between Quebrada de Tallo and Canegua. The Bu-III sequence is characterized by intercalations of sheet-like unchannelized mud-supported debris flow deposits and mud-dominated sand-mud couplets (C2.4). The matrix content is greater than 50% and the components consist exclusively of reworked sandstones and muds. The maximum clast size of the sandstones is c. 20 cm in diameter and c. 50 cm for the mud-clasts. A prominent feature of the upper part of the sequence is a large-scale slump that reaches a thickness of c. 300 m. The deformed strata consist of mud-dominated sand-mud couplets similar to the underlying deposits and medium-bedded sand-mud couplets (C2.2). The internal structure is characterized by large-scale coherent folding. Neither dislocated nor brecciated strata have been observed within the sediment slide. However, brecciated and balled strata layers of 10 and 5 m, respectively, occur beneath the base and on top of the sediment slide. This sequence contains an ichnoassemblage which includes Chondrites, Helminthopsis and Teichichnus, and is characterized by non-vagile, deep deposit-feeder structures .

Bu-IV sequence

The outcrops of the Bu-IV sequence are located in the north of Canegua up to Quebrada Corotu. The sequence starts with 200m of thin-bedded sand- mud couplets (C2.3) which are organized in highly regular fining-upwards cycles (2- 5m cycle length). Upsection,

common intercalations of disorganized gravels (Al.l),

muddy gravels (A1.2), gravelly muds (A1.3), pebbly

sandstones (Al.4) and inversely graded gravels which

pass into stratified pebbly sandstones (A2.2-A2.5)

occur. These coarse-grained deposits are

unchannelized and up to 3m in thickness. Clasts

consist of andesites, basalts, tuffs, Iimestones and

radiolarites. The clast-supported gravels and pebbly

sandstones prevail in the lower part, whereas the

upper part is characterized by thick-bedded graded­

stratified siltstones. Occasionally intercalations of mud­

supported debris flow deposits occur, mainly

containing sandstones and mud clasts. The occurrence

and distribution of trace fossils within this sequence is

similar to the Bu-II sequence.

LITHOLOGY: COMPONENTS:

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Fig. 10.4. Legend to columns 1- 15 of Fig. 10.5.

Conclusions The turbidite section of the Burica Member is

inte rpreted to represent trench-slope deposits (Figs

10.4 and 10.5). Two major facies associations can be

distinguished.

1. Slope basin association. The slope basin

association occurs in the lower part of the

section. The facies association comprises coarse­

grained channel-fill deposits, sand-rich channel­

fill deposits with associated overbank deposits

and tabular lobe deposits. The compositional

petrography of the channel-fill systems shows

that the mate rial derived from local basic igneous

complex sources (underlying basement), whereas

the overlying sandlobes reveal arc-derived

material.

2. Slope association. The slope association of mud­

dominated sand-mud couplets. In the lower part

sediment slides and internally dislocated,

brecciated and balled strata are intercalated,

whereas upsection interca la tions of disorganized

gravels, muddy gravels, gravelly muds and

pebbly sandstones prevail. Detrital components

show arc-derived material. Palaeobathymetric

estimations from foraminifera indicate a

shallowing of depositional palaeodepth from

2000 to 1200m during the Pliocene (Corrigan et

al. 1990). Shallowing is also expressed by the

vertical facies distribution of turbidite systems.

The vertical occurrence and distribution of

ichnoassemblages along with the increase of the

diversity of ichnospecies w ithin the upper part of

the section may also support the gradual

shallowing of the depositional environment. The

facies architecture of turbidite systems favours

eustacy as the main Controlling factor. Sequence

boundaries are indicated by major facies changes

and an abrup t increase of the sand-mud ratio.

The tre nch-parallel slope basin was bounded by a

structural uplift which shie lded the basin from

arc-derived terrigenaus material. The occurrence

of arc-derived material in the depositional

sequence Bu-11 indicates the deactivation of the

structural high and backcutting of the feeder

channel. The subsequent deposition of thick

sequences of slope sediments reveals a

depositional slope environment, which was

probably fed by a delta system.

Acknowledgements Financial support by the Deutsche Forschungs­

gemeinschaft (DFG, Se 490/ 1-3) is g ratefully

acknowledged. We thank H. Seyfried (Stuttgart) for

encouragement and his continuous in te rest in this

project and K.T. Pickering for critical comments on the

manuscript. Thanks are also due to R. Rath (Stuttgart)

for the illustrations.

Heferences Amann, H. 1993. Randmarine und terrestrische

Ablagerungsräume des neogenen Inselbogensystems in Costa Rica (Mittelamerika). Profil, 4, 1- 161.

Burbach, G., Frohlich, C., Pennington, W. and Matumoto, T. 1984. Seismicity and tectonics of the subducred Cocos Plate. Journal oj Geophysical Research, 81, 31-63.

Corrigan, ]. , Mann, P. and Ingle, Jr, J.C. 1990. Foreare response to subduction of the Cocos Ridge, Panama-Costa Rica. Bulletin ofthe Geological Society oj America, 102, 628-652.

Eynatten von H., Schmidt, H. and Winsemann, ]. 1992. Sedimentation history and geodynamic significance of Plio-Pleistocene shallow and deep marine forearc sediments (Osa Peninsula, Costa Rica). Zentralblatt für Geologie und Paläontologie, Teil I, 191, 1479-1492.

Eynatten von, H., Krawinkel, H. and Winsemann, ]. 1993. Plio-Pleistocene outer arc basins in southern Central America. In: Frostick, L. and Steel, R. (eds) Sedimentation a nd Tectonics. Special Publications of the International Association of Sedimentologists, 20, 39~14.

Kolb, W. and Schmidt, H. 1991. Depositional sequences associated with equilibrium coastlines in the Neogene of southwestern Nicaragua. In: MacDonald, D.I.M. (ed.) Sea­level Changes at Active Plate Margins: Processes and products. Special Publications of the International Association of Sedimentologists, 12, 259- 272.

Lonsdale, P. and Klitgord, K.D. 1978. Structure and tectonic history of the eastern Panama Basin. Bulletin oj the Geological Society oj America, 89, 981-999.

Lowrie , A. , Aiken, T. , Grim, P. and McRaney, L. 1979. Fossil spreading centre and faults within the Panama fracture zone. Marine Geophysical Researches, 4, 153-166.

MacKay, M.E. and Moore, G.F. 1990. Variation in deformation of the South Panama accretionary prism: response to oblique subduction and trench sediment variation. Tectonics, 9, 683-698.

Pickering, K., Stow, D., Watson, M.P. and Hiscott, R. 1986. Deep-water facies, processes and models: a review and classification scheme for modern and ancient sediments. Earth-Science Reviews, 23, 75-174.

Schlegel, R. 1992. Profilaufnahme und sedimentalogische Untersuchungen der plio-/ pleistozänen Tiefwassersedimente der Halbinsel Burica (Costa Rica, Panama). Diplomarbeit, Universität Stuttgart.

Schmidt, H. and Seyfried, H. 1991. Depositional sequences and sequence boundaries in fore-arc coastal embayments:

case histories from Central America. In: Macdonald, D.I.M. (ed.) Sedimentation, Techtonics and Eustasy; Sea­level changes at active margins. Special Publications of the International Association of Sedimentologists, 12, 241- 258.

Seyfried, H., Astorga, A., Amann, H., Calvo, C. , Kolb, W., Schmidt, H. and Winsemann, ]. 1991. Anatomy of an evolving island arc: tectonic and eustatic control in the south Central American forearc area. In: Macdonald, D.I.M. (ed.) Sedimentation, Techtonics and Eustasy; Sea­level changes at active margins. Special Publications of the International Association of Sedimentologists, 12, 217- 240.

Winsemann,]. and Seyfried, H. 1991. Response of deep­water fore-arc systems to sea-level changes, tectonic activity and volcaniclastic input in Central America. In: Macdonald, D.I.M. (ed.) Sedimentation, Techtonics and Eustasy; Sea-level changes at active margins. Special Publications of the International Association of Sedimentologists, 12, 273-292.

Winsemann, ]. 1992. Tiefwasser-Sedimentationprozesse und -produkte in den Foreare-Becken des mittelamerikanischen Inselbogensystems: eine sequenzstratigraphische Analyse. Profil, 2, 1-218.

Winsemann, ]. 1993. Historia del antearco mesoamericano austral y sus implicaciones para Ia evolucion de Ia placa del Caribe. Revista Geol6gica de America Centrat , 16, 1-20.

Wortmann, U. 1992. Das Profil des Burica Schichtgliedes in West-Panama. Diplomarbeit, Universität Stuttgart.

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