ABSTRACT

The role of storms is of major importance in forming stratification in open-shelf facies. Though storm processes on modern shelves are not fully understood, enough is known to see how sediments are influenced. Storms transport sand away from coastal areas, and suspend and transport sand and mud on the open shelf when wave base is lowered. Storms generate winnowed lag deposits of shells or gravel, but coarser sediment and shells of megafauna are little transported except in very near shore environments.

The Martinsburg Formation (Middle and Upper Ordovician) of southwestern Virginia was deposited on a storm-influenced open-marine platform, in contrast to the classic turbidite flysch basin of the Martinsburg in the Great Valley of Virginia to the northeast. Deposition of most Martinsburg sediments was due to storms. Sediments deposited during single storm events occur in characteristic fining-upward sequences (1 to 80 cm thick) in both carbonate and terrigenous clastic dominated areas of the platform. Individual storm deposits consist of a sharp erosional base overlain by a couplet of whole-fossil packstone and laminated calcarenite/siltite or quartzose siltstone to very fine sandstone, which is gradational to shale. Packstones are typically intraclastic, have shells aligned parallel o bedding and convex up (often with adhering or contained mud), and have abundant infiltration textures such as cement filled shelter-voids. Packstones grade abruptly to a laminated bed with horizontal plane lamination, large- and small-scale in-phase undulatory lamination, and wave-ripple cross lamination, commonly occurring in ordered vertical sequences. Laminated beds contain escape burrows, are weekly graded, and have irregularly mounded upper bed surfaces. Laminated beds grade upward to shale containing horizontal branching burrows and marine fossils. The faunal content of the fining-upward sequences matches that of the interbedded shale. Storm-generated beds vary greatly in thickness laterally, and frequently pinch-out completely.

Fining-upward sequences formed by scouring, winnowing, and suspension of the sand and mud during the height of the storm to form the erosional base and autochthonous lag of whole fossils. Storms preferentially preserve whole fossils rather than fragmenting them. Infiltration of finer sediment into the lag deposit and deposition of the laminated bed occurred as the storm waned. Laminated beds are molded into a distinctive suite of sedimentary structures by wave-generated currents. The shale represents deposition of finest storm-suspended material and continued hemipelagic sedimentation. Similar sequences are widely distributed in the geologic record.

Variability of storm-generated sequences can be related to changing water depth. Thus storm deposits may be useful in basin analysis to estimate relative water depths and trace transgressive and regressive events.