Depositional response to Quaternary fourth-order sea-level falls on the Latium margin (Tyrrhenian Sea, Italy)
Francesco L. Chiocci, 2000. "Depositional response to Quaternary fourth-order sea-level falls on the Latium margin (Tyrrhenian Sea, Italy)", Sedimentary Responses to Forced Regressions, D. Hunt, R. L. Gawthorpe
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More than 10 000 km of high-resolution seismic profiles permit detailed study of six fourth-order seismic stratigraphic sequences deposited during the last million years on the Latium continental shelf, Tyrrhenian Sea, Italy. Sedimentation occurred on a relatively young passive margin characterized by a narrow and relatively steep shelf where sediment storage capacity in adjacent subaerial basins was limited. The Late Pleistocene deposits are locally tilted and eroded to different levels along the sea-floor so that carefully placed seafloor gravity cores help constrain the age of the seismic sequences by the dating of microfauna. Correlation with a deep well located on the coast constrains the basal thirdorder sequence boundary on the shelf. The seismic and a limited core database make it possible to: (1) detail variability in the architecture, stratal patterns and bounding surfaces of the sequences across the shelf and adjacent continental slope; (2) define a hierarchy of seismic units and their bounding surfaces; (3) make a correlation with the published oxygenisotope curves; (4) develop a detailed stratigraphic framework and model for the fourthorder sequences deposited during last 0.8 Ma; (5) define the effects of the longlasting eustatic falls on margin sedimentation; (6) recognize volumetric partitioning of sedimentation between systems tracts.
The seismostratigraphic expression of the third and fourthorder sequence boundaries varies greatly from the inner to the outer part of the margin. Where subsidence allowed the preservation of lowstand systems tract (LST) deposits on the shelf, they are bounded by erosional unconformities interpreted to reflect fourthorder glacioeustaticallydriven cycles. Relatively thin (<10 m) lensshaped bodies mark the transition from the unconformities to their correlative conformities and are interpreted to have been deposited during the eustatic minimum. The deposits bounded between correlative conformities show an upward loss in acoustic transparency thought to indicate upwardcoarsening and regression within the sequences. Downdip on the continental slope, sequence boundaries are concordant surfaces correlative with unconformities on the shelf. However, these surfaces are locally scoured by channellized features, interpreted to record slope erosion related to the discharge of river bedload during lowstands.
There is marked asymmetry and volumetric partitioning between systems tracts; most of the Late Quaternary deposits that comprise the Latium continental margin are interpreted to have formed during forced regression and lowstand. Offshore of the northern and central Latium shelves forced regressive and lowstand deposits account for some 1000 km3 of shelf and slope deposition during the last eustatic cycle. In contrast, sediments attributed to the transgressive and highstand systems tracts account for approximately 37 km3. Highfrequency, highamplitude asymmetric sealevel changes driven mainly by glacioeustasy are interpreted to have controlled deposition. Following classic threefold sequence stratigraphic models, the unconformities created by shelf subaerial exposure and erosion represent sequence boundaries at the base of depositional sequences. However, if as is the case of the Latium margin during the Late Pleistocene, where a continental margin is formed almost exclusively of forced regressive deposits, each sequence basal boundary will paradoxically be situated above the forced regressive deposits that are deposited as the subaerial exposure surface forms, i.e. above the whole of its own depositional sequence. In this respect, the incorporation of a fourth forced regressive or falling stage systems tracts specific to times of baselevel fall would help avoid this inconsistency.
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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.