Response of Plio-Pleistocene mixed bioclastic–lithoclastic temperate-water carbonate systems to forced regressions: the Calcarenite di Gravina Formation, Puglia, SE Italy
Marcello Tropeano, Luisa Sabato, 2000. "Response of Plio-Pleistocene mixed bioclastic–lithoclastic temperate-water carbonate systems to forced regressions: the Calcarenite di Gravina Formation, Puglia, SE Italy", Sedimentary Responses to Forced Regressions, D. Hunt, R. L. Gawthorpe
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Upper Pliocene-lower Pleistocene shallow-marine temperate-water carbonates of the Calcarenite di Gravina Formation crop out in the Murge area of Puglia, SE Italy, and record a regional subsidence-driven transgression that was punctuated by higher-frequency forced regressions. Sedimentation occurred during the drowning of a complexly faulted island archipelago whose bedrock was exclusively composed of deformed Cretaceous platform carbonates. High-energy temperate-water bioclastic carbonate systems dominated marine environments, but bioclasts were locally mixed with carbonate lithoclasts derived from the Cretaceous limestones bedrock and supplied to the shoreline via ephemeral rivers. This setting allows us to compare the depositional response of bioclastic-dominated and mixed bioclastic-lithoclastic temperate-water carbonate systems to relative sea-level changes, and in particular to forced regressions within a long-term transgressive sequence set.
Bioclastic dominated temperate-water carbonate systems are comprised of a nearshore non-depositional abrasion zone and an offshore accumulation zone; long-term subsidence led to erosional transgression through nearshore abrasion and bioerosion of the drowning archipelago. The bioclastic-dominated carbonate system was best developed during relative sea-level rises and highstands, with offshore cyclic subtidal carbonate successions interpreted to record higher-frequency relative sea-level fluctuations. Forced regressions and lowstands were associated with basinward migration of the abrasion zone and development of a subaerial exposure surface that passed basinward into marine rock- and softgrounds on the shelf; little additional sediment was supplied from updip karstic areas of the island archipelago where superficial drainage was limited.
In contrast, mixed bioclastic-lithoclastic carbonate systems are characterized by reciprocal sedimentation, developed where ephemeral rivers supplied carbonate lithoclasts to the shoreline. In these systems, bioclastic sedimentation typified relative sea-level rises and highstands whereas forced regressions and lowstands were associated with the development of coarse lithoclastic deposits. Forced regressive-lowstand deposits are represented by narrow progradational gravel beaches in ramp settings whereas small coarse-grained deltas formed against steep fault-bounded coastlines; both lack an aggradational component. Lower surfaces of the forced regressive-lowstand units are sharp and record abrupt basinward facies shifts. However, these basal surfaces were largely inherited, formed in the nearshore abrasion zone of the preceding transgressive-highstand bioclastic-dominated carbonate system. Rockgrounds formed in this way were not substantially modified by marine shoreface erosion during sea-level fall. The upper bounding surfaces of the forced regressive/lowstand deposits are also marine in origin and developed in response to rapid sea-level rise and landward translation of the shoreline. These surfaces were associated with nearshore abrasion and ravinement so that subaerial exposure surfaces were reworked in the marine environment and have very low preservation potential. Accordingly, the forced regression/lowstand sediment bodies are bounded by marine erosion surfaces and enclosed within sediments and/or surfaces formed in offshore environments.
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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.