Recognition of ancient tidal inlet sequences: an example from the Upper Silurian Keyser Limestone in Virginia

Abstract

ABSTRACTThe Upper Silurian Keyser Limestone is a relatively thin (< 85 m) unit of lagoonal, barrier, and shallow offshore sediments that crops out in the central Appalachians. Lithologies include massive micritic limestones to calcarenites, calcisiltites, and calcareous quartz arenites. The barrier lithofacies is preserved predominantly as tidal inlet channel‐fill. Its presence is supported by two lines of evidence: (1) the sequence of sedimentary textures and structures resembles that observed in modern inlets, and (2) the sequence occupies a position immediately above a disconformity, and is accompanied by an abrupt vertical change in faunal diversity, which is interpreted as representing the transgression of open marine over back‐barrier environmentsThe inlet channel sequence comprises fine‐ to medium‐grained, well‐sorted quartz arenites that disconformably overlie sediments deposited on carbonate tidal flats (laminated, mudcracked pelmicrites). The sandstone displays a fining‐upward texture, and contains a broken and abraded mixed fauna. Cross‐bedding is bipolar, with major modes oriented obliquely to depositional strike. Decimetre‐scale sets of planar and trough cross‐beds grade upward to centimetre‐scale sets of ripple cross‐lamination, washed‐out ripples, and plane beds. This sequence represents the change from deep to shallow channel environments, and is attributed to lateral inlet migration. The inlet sequence was preferentially preserved during marine transgression because of its relative thickness and lower stratigraphic position with respect to overlying and adjacent barrier‐beach sediments.The vertical relationships of this inlet‐lagoon complex emphasize that care must be taken in interpreting shallow‐water transgressive sequences. Vertical ‘jumps’ in faunal diversity accompanied by scour surfaces could be misconstrued as major unconformities. Instead, such sequences may represent the shoreface erosion normally associated with the transgressive migration of barrier islands. Whether or not the faunal jump is accompanied by a barrier lithosome is greatly dependent on the geometry, frequency, and migration rate of tidal inlets.