Abstract: Recognition of Cratonal Bank, Slope, and Basinal Facies, Ordovician Trenton-Utica Sequence of New York

Barry Cameron, Mohammed Tahir Bukhari

The middle Trenton Group in its type area in central New York changes facies along a northwest-southeast direction from shallow epeiric sea carbonates to basinal Utica black shales. This major facies changes within the craton over a relatively short distance (10 to 15 km) across a northeast-southwest bank margin.

From the bank to basin (eastward), the dominant rock types change from interbedded limestones and calcareous shales to black shales. Along this gradient, the limestones change from mixed mudstones, wackestones, packstones, and grainstones to argillaceous mudstones, the latter occurring along the lower slope and interbedded with relatively thick black shales.

Primary sedimentary structures of the Trenton bank carbonate rocks (e.g., pararipples, current laminations, mudcracks, channels, imbricated flat pebbles, and vertical burrows) markedly decrease in occurrence eastward, while horizontal bioturbation replaces them. However, deeper slope mudstones contain different burrows, no significant bioturbation, and a return to current laminations, but with fine-grained graded lamina possibly owing to density currents.

Faunally, species diversity decreases and equitability increases across the bank margin and eastward into deeper water. The shallow-shelf benthic shelly communities are replaced by a mixed benthic and pelagic shelly community along the deeper shelf margin. The lower slope, in turn, is dominated by pelagic arthropods, whereas the black shales farther out in the basin are dominated by graptolites.

Composition, textures, sedimentary structures, and faunas change abruptly across a bank margin into deeper water, so that, with a pluralistic approach, such paleoenvironmental gradients should be readily recognizable in the stratigraphic record.

AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma