Anatomy of Extremely Thin Marine Sequences Landward of a Passive-Margin Hinge Zone: Neogene Calvert Cliffs Succession, Maryland, U.S.A.

Abstract

ABSTRACT Detailed examination of Neogene strata in cliffs 25-35 m high along the western shore of Chesapeake Bay, Maryland, reveals the complexity of the surviving record of siliciclastic sequences 150 km inland of the structural hinge zone of the Atlantic passive margin. Previous study of the lower to middle Miocene Calvert (Plum Point Member) and Choptank Formations documented a series of third-order sequences 7-10 m thick in which lowstand deposits are entirely lacking, transgressive tracts comprise a mosaic of condensed bioclastic facies, and regressive (highstand) tracts are present but partially truncated by the next sequence boundary; smaller-scale (fourth-order) cyclic units could not be resolved. Together, these sequences constitute t e transgressive and early highstand tracts of a larger (second-order Miocene) composite sequence. The present paper documents stratigraphic relations higher in the Calvert Cliffs succession, including the upper Miocene St. Marys Formation, which represents late highstand marine deposits of the Miocene second-order sequence, and younger Neogene fluvial and tidal-inlet deposits representing incised-valley deposits of the succeeding second-order cycle. The St. Marys Formation consists of a series of tabular units 2-5 m thick, each with an exclusively transgressive array of facies and bounded by stranding surfaces of abrupt shallowing. These units, which are opposite to the flooding-surface-bounded regressive facies arrays of model parasequences, are best characterized as shaved sequences in which only the transgressive tract survives, and are stacked into larger transgressive, highstand, and forced-regression sets. Biostratigraphic analyses by others indicate that this onshore record contains the same number of third-order ( 1 my duration) units as present offshore, and so thinning landward of the hinge zone was accomplished not by omission or erosion of entire cycles of deposition, but instead by omission of some subsidiary elements (e.g., lowstand tracts), by erosional shaving of sequence tops (removing the entire regressive tract in some sequences), by a reduced number of component high-order cycles surviving per larger set, and by qualitative changes in the anatomy or composition of elements (e.g., condensed transgressive tracts; shaved sequences rather than parasequences). All of these differences can be attributed to limited accommodation, ut preservation of an onshore record of each baselevel cycle was probably also favored by the large amplitude and rapidity of eustatic fluctuations during the Miocene.