Late Quaternary sedimentation and high-resolution sequence stratigraphy of the east Texas shelf

Abstract

An ongoing study utilizes outcrop-scale seismic data and lithofacies data from cores and platform borings collected from the east Texas continental shelf to test assumptions and models that relate sedimentary facies patterns and sequence stratigraphy. The current data base consists of nearly 15 000 km of high-resolution seismic data and lithological data from hundreds of sediment cores and platform borings. The Texas shelf is ideally suited for this work because sediment supply during the Pleistocene was high enough to keep pace with the relatively rapid rise and fall in sea level, thus producing sequences that can be imaged on highresolution seismic records. Furthermore, sediment supply, shelf gradient, and the degree of diapirism and faulting vary across the shelf, so the relative roles of these agents in controlling the overall packaging of facies into systems tracts can be evaluated.Strong contrasts exist between low-sediment-supply (Trinity/Sabine) and high-sedimentsupply (Brazos and Colorado) fluvial systems and their associated systems tracts. The predominant difference is that sediment delivered to the shelf by the Trinity/Sabine fluvial system was, for the most part, deposited within the incised fluvial valleys. Only during lowstands did the Trinity/Sabine system deliver sediment directly to shelf-margin deltas and slope minibasins. Reincision, associated with fifth-order eustatic fluctuations, flushes the valley of sediments deposited during the previous transgression and highstand, thus large quantities of sediment are delivered from this valley system during lowstands. Transgressive shelf sand bodies occur adjacent to the Trinity and Sabine incised valleys and are scattered widely across the shelf. Backstepping parasequences, the product of the episodic nature of glacial eustatic sea-level rise, characterize the incised valley fill. The highstand systems tract is thin to absent in interfluve areas.The Brazos and Colorado rivers have much larger sediment supplies than the Trinity and Sabine rivers. They have filled their incised valleys with fluvial deposits and abandoned them to occupy several more shallow valleys. The result of this fluvial avulsion has been the sequestering of a significant part of the sediments delivered by these rivers to fluvial valleys on the shelf. Both the Colorado and Brazos rivers constructed large shelf-margin deltas during the lowstand, but these deltas differ in terms of their overall morphology and sediment facies. The ancient Colorado delta is sandy and it directly sourced two slope fans during the maximum lowstand. The Brazos shelf-margin delta consists mainly of mud and there is little evidence of significant bypass during the lowstand. During initial transgression, the Brazos/Colorado shelf-margin deltas backstepped onto the outer shelf. Rapid transgression and associated erosion removed the delta-plain beds. Continued transgression led to decapitation of sandy fluvial and deltaic facies, reworking the sands into widespread shelf sand bodies, and further backstepping...

Geomorphology and age of the Oxygen isotope stage 2 (last lowstand) sequence boundary on the northwestern Gulf of Mexico continental shelf

“…8). This shoreparallel orientation of these valleys was influenced by presence of an older delta, the Western Louisiana Delta, which lies above the OIS5e maximum flooding surface but below the OIS2 sequence boundary (Anderson et al 1996;Wellner et al 2004), on the outer shelf. This demonstrates the importance of antecedent topography in controlling the flow directions of rivers.…”

“…On the outer shelf, seaward of where these valleys merge, platform borings sampled an average of 30 m of sand within the valley . At the shelf break, the valley is linked to an incision in the upper slope that is situated within a salt-withdrawal minibasin (Anderson et al 1996). On the inner shelf, the Brazos valley has a terraced morphology, which indicates that it was initially cut during the falling limb of sea level (Anderson et aL 1996;Abdulah et al 1997Abdulah et al , 2004.…”

Late Quaternary shelf-margin wedges and upper slope progradation in the Gulf of Cadiz margin (SW Iberian Peninsula)

“…Shelf facies are generally attributed to deposits generated during episodic sea-level falls (Sydow & Roberts 1994;Anderson et al 1996), whereas upper slope sedimentation is especially favoured during lowstand and early rise conditions (Kindinger 1988;Anderson et al 1996). 2 Thickness increase towards the upper slope is another indicator of their relationship with relative sea-level falls (Anderson et al 1996). 3…”

“…Erosional unconformities at the top boundaries are related to subaerial erosion during sea-level falls (Kindinger 1988;Anderson et al 1996), which may cause the removal of sediments deposited during the early stages of forced regressions (Chiocci 2000).…”

“…Shelf-margin wedges are thus interpreted as prograding complexes of the LST, formed during periods of falling sea-level and lowstand (Piper & Perissoratis 1991;Ercilla et al 1992Ercilla et al , 1994aMorton & Suter 1996;Chiocci et al 1997;Yoo & Park 1997). However, other studies consider two main constructional phases of shelf-margin deltas, which show an overall configuration of progradation-aggradation (Kindinger 1988;Sydow & Roberts 1994;Torres et al 1995;Anderson et al 1996;Hiscott 2001) related to sea-level fall/lowstand conditions (Kolla et al 2000):…”