Box Sequences and fossils

There are eight brachiopod-dominated biofacies recognized across an onshore-offshore gradient in the Upper Ordovician rocks of Kentucky (Holland & Patzkowsky 2004). These assemblages were not discrete but rather formed part of a depth-related gradient, and the relative abundance of species varied through time. The development of these faunas across this part of the Appalachian Basin can be charted within sequence-stratigraphic frameworks. Figure 2.11 is a plot of the DCA (detrended correspondence analysis) axis 1 against the litho- and sequence stratigraphy of one of the key sections, the Frankfort composite section. The DCA axis is a proxy for taxa that were grouped together in the shallowest-water environments. Thus within the highstand system tracts, values for this axis are lower than those for the transgressive and system tracts and at the maximum flooding surface, where deeper-water taxa dominate. The upsection faunal changes show that the distribution of taxa was controlled by ecological factors dependent on sediment supply and sea-level changes, which in turn built the sequence stratigraphic architecture. Marked fluctuations in the faunas occurred during net regressive and transgressive events, emphasizing the depth-dependence of these assemblages.

The data used in this study are available at http://www.blackwellpublishing.com/paleobiology/.

Flooding surfaces

Clays Ferry

0 0

Post a comment