6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 588-591
Long-term tectonic influence on sequence architecture and stacking pattern revealed by hiatal shell bed features: Pleistocene succession of the Canoa Basin (central Ecuador) Luca Ragaini
1,
Claudio Di Celma
2,
Gino Cantalamessa
2,
a Walter Landini 1
1 Dipartimento di Scienze della Terra, Università degli Studi di Pisa, Via S. Maria 53, Pisa, Italy. Email addresses:
[email protected]. it;
[email protected] 2 Dipartimento di Scienze della Terra, Università degli Studi di Camerino, Via Gentile III da Varano, 1 - 62032 Camerino (MC), Italy. Email addresses:
[email protected];
[email protected]
Introduction
Taphonomic and paleoecological analyses of condensed shell concentrations may provide a means of identifying discontinuities and stratal surfaces where these are obscure (e.g., Beckvar and Kidwell, 1988). ln recent years several authors have highlighted the importance, in conjunction with traditional lithofacies analysis, of taphonomic and paleoecological criteria in outcrop-based sequence stratigraphie studies. These allow for better resolution of sequence subdivisions of siliciciastic successions, and identification of their internai architecture at a more detailed scale than possible with sedimentary facies alone (e.g., Abbott and Carter, 1994; Naish and Kamp, 1997; Carter et al., 2002; Di Celma et al., 2002; Cantalamessa et al., 2005). This is particularly true for Plio-Pleistocene successions where, generally, a very large number of taxa recovered from the fossil assemblages are still extant and, therefore, significant and reliable details of their ecology are more easily available. Based on outcrop data from the southern coast of Cabo San Lorenzo (Ecuador) (Fig. 1), facies, shell bed features, and sequence stratigraphie framework for the shallow-marine Pleistocene upper Canoa and Tablazo Formations are presented (Fig. 2). Sediments of this succession exhibit a distinct cyclic pattern consisting in a stack of eight depositional sequences Iikely developed under the main control of orbitally-induced sea-Ievel changes. The large-scale stratal architecture of the entire succession has been strongly influenced by the continuous uplift of the Cabo San Lorenzo area. Each successively younger sequence dips to the south-southwest at a shallower angle from a maximum of 5° for the oldest to JO for the youngest, clearly indicating synsedimentary tilting and depocentre migration. This pattern generated discernible discordances between successively younger depositional sequences and a remarkabJe southward expansion of the basin-fill associated with the southward divergence of sequence-bounding unconformities. As a mie, within the studied interval an idealized sequence is composed of a transgressive (TST) and a highstand (HST) systems tract, whereas deposits attributable to the lowstand and falling-stage systems tracts are missing. Transgressive lithosomes may be defined by estuarine deposits interposed between the sequence boundary and the ravinement surface (backbarrier wedge) and by upward fining shoreface to inner-shelf facies successions above the ravinement (backstepping shelf wedge). Separated by an expanded siliciclastic core, hiatal shell concentrations occur at the base (onlap shell beds, OSB) and the top (backlap shell beds, 8SB) of the transgressive shelf wedges and, sometirnes, at the base of the highstand systems tracts (downlap shell beds, DS8). The primary and specifie aims
588
6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 588·591
of this study are : (1) to understand how the shoreline configuration infIuenced sheII bed features, the nature of other architectural elements and, consequently , the internai architecture of depositional sequences; (2 ) to provide new insights into the control played by synsedimentary tectonic mechanisms on the shoreline configuration and the vertical arrangement of shallow-marine clastic sequences .
Figure 1. Structural map of the Ecu adorian coast showing position of the study area (boxed part) with respect to the place where the Carnegie Ridge impinges on the Ecuador Trench
Influence of the coastline configuration on the internai anatomy of depositional sequences
Several factors, such as rate of relative sea-level rise , coastline configuration du ring transgression, and the volume and grain size of sediment suppl ied, influence the thickness , grain size, sedirnentary structure, grade of bioturbation, taphonomy, palaeoecology and other Ieatures of OSBs. Among ail, coastline configuration acts as an important factor in that it con trois local energy conditions on which depends the efficiency of sediment bypassing and therefore the net sedimentation rate at the shoreline. Onlap shell beds form above the storrn-wave base during transgressions and, because of the high environmental energy of this setting, commonly are sedimentologic accumulations developed by the extensive reworking of autochthonous communities under sediment bypass conditions. Nevertheless, within the upper Canoa Fm OSBs are infaunal-dominated community beds associated with shoreface sediments. The absence of primary sedimentary structures, entirely removed by the intense bioturbation, and the accumulation of infaunal-dominated community shell concentrations, argues for sediment deposition within relatively sheltered shorefaces where wave action and, consequently, sediment bypass were minor. Unlike sequences of the upper Canoa Fm , those of the Tablazo Fm are dominated by OSBs formed through repeated reworking within high-energy settings and suggest accumulation within more exposed, storm-infIuenced shorefaces, where wave action and sediment bypass were more intense. The existence of these two well-distinguished energy regimes within the seq uences of the upper Canoa and Tablazo Formations is also confirmed by the different set of component facies, and the type of vertical transition that exists, within the late transgressive sediments, between the normally supplied nearshore facies and the overlying sediment starved, inner-shelf BSBs. The graduai transition recorded within the sequences of the upper Canoa Fm reflect reduced
589
6th Internat ion al Symposium on Andean Geodynamics (ISAG 2005 , Barcelona ), Extended Abstracts: 588-591
wave energy and, possib ly, higher rates of sedime nt s upply whereas the sharp, erosive transition reco rded wi thin the cycJothems of the T ablazo Fm , reflect mor e ene rge tic, storrny inner-shelf settings accompanied by for mat ion of an erosive local tlooding surface (LFS) . Such vertical changes in geometrie features and mode of shell concentration both in shoreface and inner-she lf settings, is cle arly due to the progress ive upl ift of the Canoa Basin and the consequent fo rma tio n of a Jess dee ply ernbayed, more ex po sed con figuratio n by successive intergl acial coastlines .
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Figure 2. Composite strat igrap hie sec tion co mpi led for the Pleistocen e sed imen tary succes sion ex pos ed betwee n El Mangle (Pi le) and Rio Ca llejo n (San José). The eight deposition al sequences identified are subdivi ded in the co mpo nent systems tract s. BBW , back-ba rrier wedge: BSW , backstepping shelf wedge ; TST , transgressive sys tems tract ; HST , high stand systems tract.
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6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona), Extended Abstracts: 588-591
Sequence pattern through tirne
The stratal architecture of the eight dep osit ion al seque nces described in this paper and the way they are stacked, pro vide a record of the dyn amic interplay of high -frequency sea -Ievel changes, tectonism , chan ging sho reline configuration , and sedi ment supply. At a multi-cycle time scale, tectonics influenced the lon g-term trend of the relative se a- Ieve l changes and , con sequentJy , the large-scale stratigraphie or ganization, which displ ays a distin ct ive s tac king pattern . Th e sea ward and do wnward retreat of successive maximum floodin g shorelin es, as weil as the progressive incr ease in grain size of the sedimentary facie s en casing the BSB s (i.e. compared to tho se of the upper Cano a Fm, BSB s of the Tablazo Fm accumulated in a more proximal setting) , suggest that the sequence set represents a fall ing-sta ge se quence set. The long-term trend of relati ve sea- Ieve l fall that has dri ven the forced regre ssion is the product of tecton ic deformation in the San Lorenzo are a. Thi s requires syn-sedimentary uplift that progressively subtracted accommodation for sediments From north to south. and produced a so uthward divergen ce of sequence-bounding unconformities, the progressive rotation of the o lder sequences, and hence the stratigraphie ex pansion of the succession in this direction .
References Abbott, S T ., and Carter, R.M., 1994, The sequence architecture of mid-Pleistocene (c.l.I -OA Ma) cyc lothems from New Zealand : faci es dev elopment during a period of orbital control on sea-\evel cyclicity, in de Boer, P.L.,
and Smith , D.G ., eds.,
Orbital forcing
and
cyclic sequences: International
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Sedimentologists Spe cial Publi cation 19, p. 367 -394. Beckv ar, N., Kidwell, S., 1988, Hiatal shell concentrations, sequence analysis, and sea -Ievel history of e Pleistocene coastal alluvial fan , Punta Chueca, Sonora: Lethaia, v. 21, p. 257-270. Cant alarne ssa, G ., Di Celma , C; Ragaini , L., 2005, Sequence stratigraphy of the middle unit of the l am a Form ation
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Palacoecology, v. 216, p. 1-25 . Carter, R.M ., Abbott, S.T., Gr aham , 1.1 ., Nai sh, T .R., Gammon, P.R ., 2002 , The middle Pleistocene Merced-2 and -3 sequences From Oce an Bea ch , San Francisco : Sedimentary Geology , v. 153, p. 23 -41. Carter, R.M ., Naish , T.R. , 1998, A review of Wan ganui Basin , New ZeaJand : global reference sectio n for shallow marine, Plio-Plei stocene (2.5--0 Ma) cyclostrati graphy : Sedimentary Geology , v. 122, p. 37-52 . Di Celma , c., Ragain i, L., Cantalamessa, G., and Curzio, P., 200 2, Shell concentrations as tool s in characterizing sed imentary dynamics at sequence- bound ing unconformities: examples from the lower unit of the Canoa Formation (Late Pliocene, Ecuador): Geobios, v. 35, p. 72-85.
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