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The AAPG/Datapages Combined Publications Database
Journal of Sedimentary Research (SEPM)
Abstract
Anatomy of Extremely Thin Marine 
Sequences
Landward of a Passive-Margin
Hinge Zone: Neogene Calvert Cliffs Succession, Maryland, U.S.A.
Susan M. Kidwell
ABSTRACT
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.
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