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Recognition Criteria and Facies Models
An Exhumed, Lower Cambrian Tidal Flat: The Antietam Formation, Central Virginia, U.S.A.
At Natural Bridge Quarry, Virginia, tidal-flat facies are preserved within the Antietam Formation, of the Upper Precambrian-Lower Cambrian Chilhowee Group. The Antietam Formation is composed of two storm- and wave-dominated progradational parasequences; the uppermost of these is capped by a sequence boundary overlain by tidal-flat facies. Shoreface deposits are in erosional contact above and below the tidal-flat sediments.
Preserved sedimentary structures within the tidal-flat assemblage permit construction of a model for the ebb-through-flood phase. During the flood phase, flood channels and topographic highs were migration sites of small- to medium-scale, subaqueous dunes with superimposed current ripples. Current, oscillatory, and combined-flow ripples developed marginal to the flood channels, followed by suspension settling of mud during the flood stillstands. Peak ebb flow was marked by development of dunes mainly in ebb channels. Both flood- and ebb-oriented dunes and current ripples were modified during ebb runoff and emergence. Runoff along current ripple and dune troughs downcut troughs and resulted in deposition of runoff microdeltas. Near the ebb slack water, the tidal flat became emergent; lee sides and troughs of dunes were either covered with current ripples, erosively steepened, infilled with sand or crosscut by rill features.
Tidal processes developed during transgression across exposed shoreface deposits of the initial sequence of the passive margin. The tidal-flat facies was deposited probably in an estuarine setting. An upper microtidal to lower mesotidal setting is indicated by preserved paleorelief, scale of bedforms, trace fossil distribution and thickness of tidal-flat facies. High frequency sea level changes or autocyclic processes within the near-shore system are possible mechanisms that caused preservation of the bedforms. In other outcrops of the Chilhowee Group, preservation of tidal-flat deposits is rare because shoreface retreat eliminated more proximal facies, resulting in amalgamation of progradational and transgressive shoreface deposits.
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