The processes responsible for sediment deposition and the development of internal morphology of hummocky cross-stratified units (HCS) have been the subject of considerable controversy. The results of the present study of the Ithaca Member of the Upper Devonian Genessee Formation in western New York

End_Page 535------------------------------

State suggest a probable density-flow mechanism for the deposition of the sandstones which were subsequently modified to form HCS units.

A 150-m (500-ft) thick section of flat Ithaca sediments is well exposed in the Finger Lakes region where the formation can be traced for kilometers both parallel and normal to the paleoshoreline. This shallow marine sequence is composed largely of fine sandstones and mudstones deposited in an Upper Devonian epicontinental sea. Detailed examination of the sandstone beds at the base of the Ithaca Member reveals a marked transition over a distance of 40 km (25 mi) normal to the shoreline from a hummocky cross-stratified sequence within the more proximal facies to a sequence of interbedded sandstones and shales displaying turbidite features in the more distal western exposures. The section also indicates shallowing paleodepths upsection, with vertical sequences suggesting a progression of environments increasingly dominated by the encroaching shoreline of a regressing sea.

One explanation for the observed lateral transition of sedimentary features normal to paleoshoreline is that the hummocky cross-stratified beds and the turbidites were deposited concurrently. Subsequent reworking of the HCS unit by storm waves would account for its variant internal sedimentary features. This is in accord with the results presented in 1979 by Hamblin and Walker.

End_of_Article - Last_Page 536------------