Sequence Stratigraphy In Carbonate Successions Some Italian Examples

Abstract

ABSTRACT Field stratigraphic relationships observed in carbonate systems of various age (Triassic, Jurassic, Cretaceous, Eocene, Oligocene, Miocene) demonstrate that the response of carbonate platforms to relative sea-level changes can be quite diversified. Consequently, also the sequence stratigraphicorganization results different from one case to another and not so simple as suggested by current models. INTRODUCTION There is a general agreement that depositional sequences occur in most, if not all, sedimentary successions, and this makes sequence stratigraphy a nearly universally applicable technique for the study of the sedimentary record.1 Classical sequence stratigraphy states that most sediment is shed into basinal areas during Iow stands of sea level when shelves are exposed; during high stands, most sediment is thought to be trapped on the flooded inner shelf and on the aggrading coastal plain, while little escapes to the deep.2This principle of "low stand shedding" has been applied also to carbonate platforms, but, as suggested by many workers3 (see also references in 1), extant carbonate platforms clearly display the opposite trend. Shedding of sediment into the adjacent foresfope and deep basin implies also progradation of the platform. Similarly, many workers have proposed that carbonate platform progradation occurs primarily during relative high stands of sea level (see references in 3), while others suggest that significant progradation takes place during both high stands and low stands.4,5 The purpose of this article is to present evidence that carbonate sequence stratigraphy is not so simple as suggested by current models and that generalizations are dangerous and sometime fallacious. Field stratigraphic relationships observed in Italian carbonate depositional systems of various ages (Triassic, Jurassic, Cretaceous, Eocene, Oligocene, Miocene) demonstrate that the response of steep-sloped carbonate platforms to relative sea-level changes can be quite diversified. To avoid misleading interpretations, carbonate sequence stratigraphy should be carefully calibrated for each case, both in outcrop sections and seismic profiles. CARBONATE EXPORTATION:TWO DIFFERENT KINDS OF PRODUCTS Carbonate platforms build so close to sea level that even small rises and falls will alternatingly flood and expose the platform top and drastically affect the sediment production. But in which way? Unlike siliciclastics, which originate from erosion of older rocks, the majority of carbonate sediments (mud, peloids, skeletals, ooids, etc.) were born directly in the sea where they accumulate in relatively shallow-water conditions. These loose and soft sediments are frequently swept off bank by wind-generated currents and/or shed down the platform franks and adjacent basinai areas to produce wedges of unconsolidated sediments. It is obvious that this process is greatly enhanced when the platforms are flooded. The concept of high stand shedding of carbonate platforms implies that the sediment bodies on the franks and on the adjacent basin floor are largely high stand wedges. This concision, however, might be incorrect; it should be supported by the evidence that primary sediment (mud, grains) was originally loose.