Occurrence and Significance of Clay Drapes in a Large Point Bar: Jilt Quarry, Pliocene, Romania

Dan Jipa

In the coal quarry Jilt - South (Oltenia, Romania) a 15 to 18 m thick and around 1000-m wide point bar deposit is cropping out. The main features of the Jilt point bar are: erosional base marked by a thin gravel accumulation, fining upward trend from a lower coarse grained sand facies with large trough cross laminated units to an upper silty - clayey facies, eastward dipping inclined stratification and northward paleocurrent directions. This point bar is single - storied and consists of six visible laterally stacked, codirectional sets of inclined strata.

The clay drapes are several centimetres thick and can be observed as they cross obliquely most part of the point bar body. Their thickness is larger in within the upper silty - clayey facies. In the sandy, dominant point bar facies the clay drapes are of uniform thickness and display internal parallel and cross lamination (showing the same northward main paleocurrents direction). The clay drapes never reach the point bar base. They are limited - with erosional ends - at a 5 - 7m level above the inferior point bar boundary.

The clay drapes pattern indicates that the clay material comes from the marsh zone at the upper part of the Jilt point bar. On the inclined, depositional surface of the point bar the clay material have been transported along the bar by the fluvial current. The clayey material was mixed with the scarce sandy material provided by a very low coarse clastics influx, in a temporarily low energy environment. The down-dip extension of the clay drapes was erosionally stopped by the main flow transporting coarser grained sand along the alluvial channel axis.

The restraining of the clay drapes prolongation in the coarse grained part of the bar seems to be an attribute of the point bars generated by larger meandering rivers.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California