Seismic stratigraphy of the Gulf of Cádiz continental shelf: a model for Late Quaternary very high-resolution sequence stratigraphy and response to sea-level fall
F. J. Hernández-Molina, I. Somoza, F. Lobo, 2000. "Seismic stratigraphy of the Gulf of Cádiz continental shelf: a model for Late Quaternary very high-resolution sequence stratigraphy and response to sea-level fall", Sedimentary Responses to Forced Regressions, D. Hunt, R. L. Gawthorpe
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Single-channel, very high-resolution seismic profiles allow detailed study of the Late Quaternary stratigraphic architecture of the Gulf of Cádiz continental margin, Southern Spain. The Late Quaternary stratigraphy of this area comprises fourth-order Type 1 composite depositional sequences, generated by asymmetric relative sea-level changes of 100–110 ka duration. The composite fourth-order sequences consist of forced regressive, lowstand, transgressive and highstand systems tracts. Volumetrically, the forced regressive and lowstand systems tracts are the most important components. The fourth-order composite sequences are themselves comprised of composite fifth-order sequences formed in response to asymmetric relative sea-level changes with a duration of 22–23 ka. Sediments within the forced regressive and lowstand systems tracts dominate the 5th-order sequences; their transgressive and highstand deposits are either (i) perched above present-day sea-level and so not recorded in marine seismic data, (ii) restricted to outer-mid-shelf positions, or (iii) may be absent from the shelf altogether at the resolution of this study (e.g.<0.5 m thick). The fifth-order sea-level falls were themselves modulated by minor cycles, generating very high-frequency (sixth-order) sequences. These very high-order sequences are recognized for the last 80 ka bp, and their development is attributed to asymmetric relative sea-level cycles operating on time scales of: 10–15 ka (Heinrich events), 4–4.5 ka. (P cycles), 2.3–0.97 ka. (Dansgaard-Oeschger oscillations h cycles) and 500–50 a. (c cycles). We have developed a depositional model that accounts for the very high-frequency hierarchy of Late Quaternary depositional sequences observed in the Gulf of Cadiz marine seismic record and incorporates the age of well-constrained highstand coastal deposits that are exposed along the southern Iberian coastline. The model developed serves to illustrate the evolution and importance of depositional systems during falling relative sea-level and forced regression. Development of the forced regressive systems tract appears to be particularly significant within Quaternary strata. This is because the Quaternary was strongly influenced by a high-amplitude, high-frequency glacioeustatic signal characterized by rapid sea-level rises, very short highstands, and gradual relatively long-term sea-level falls suggesting that forced regressive deposits are likely to predominate in continental margin successions subject to low rates of subsidence.
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Sedimentary Responses to Forced Regressions
An increasing number of studies in recent years have demonstrated that significant progradation of shallow marine systems occurs under conditions of base-level fall. These new data are forcing many sedimentary geologists to critically re-evaluate many aspects of sequence stratigraphy relating to erosion and deposition during base-level (lake- or relative sea-level) fall, and the intrinsic link made between stratal geometries and base-level change. For the first time, this volume brings together a collection of articles that focus solely on forced regressions, providing a more complete picture of the development, formation, variability and preservation of the surfaces and deposits generated during base-level fall.
The results of the studies published here will be of interest to all geologists attempting to understand the relationship between changes in base-level and stratigraphy, and to all who use sequence stratigraphy as a method of stratigraphic correlation and interpretation at outcrop and in the subsurface.