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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 68 (1984)

Issue: 12. (December)

First Page: 1917

Last Page: 1917

Title: Shelf Sedimentation Above Storm Wave Base in Upper Ordovician Reedsville Formation in Central Pennsylvania: ABSTRACT

Author(s): John A. Conrad


Interbedded fine-grained sandstone and shale in a generally coarsening-upward sequence characterize the Reedsville Formation. Data on sedimentary structures, lithology, bedding characteristics, and fossils for eight measured stratigraphic sections indicate that most beds were deposited by occasional storm-generated currents. Storm facies exhibit (1) abundant winnowed shell lags coupled with low-angle cross-stratified finer sediment, (2) abrupt lateral thickness variation in many beds, (3) sharp, erosive upper and lower bed contacts, and (4) well-preserved, unabraded outer sublittoral benthic fauna. Hummocky cross-stratified beds are common, and in many places are associated with wave-rippled sandstones.

A shallow open-shelf environment is inferred. Storms of variable intensity and duration periodically scoured and suspended bottom sediments and deposited individual fining-upward units under conditions of strong but waning bed shear. An overall increase in the sandstone/shale ratio from bottom to top in the progradational sequence suggests gradual shallowing and more frequent storm-wave influence. Uppermost beds contain intertidal fossil communities. Paleocurrent data indicate east- and northeast-directed sediment transport.

These interpretations are not consistent with the common assumption that the Reedsville is simply the deep-basin, distal equivalent of the adjacent Martinsburg Formation. The present data suggest that Reedsville sediments accumulated on a shelf west of a structural hinge line that comprised the western margin of the thicker, deeper water Martinsburg sequence. Differential subsidence across this shelf-edge hinge line may account for the significant differences in paleobathymetry of the two formations.

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