Analysis of Upper Cretaceous Trace-Fossil Assemblages, U.S. Western Interior: ABSTRACT

Abstract

Diversity and abundance of trace fossils in strata of the Greenhorn and Niobrara cyclothems of west-central Kansas, southern Colorado, and south-central Utah have been used to quantify trace-fossil assemblages. Recognition of assemblages is based on use of quantitative, semi-quantitative, and binary (presence-absence) data in conjunction with cluster and factor analysis. Although the character of the original burrowing infauna was a primary factor in assemblage composition, the nature of burrowed sediments and diagenesis exerted strong influence on the preserved trace-fossil record. Thus, the present composition of originally similar assemblages may differ among the several lithotypes (sandstone, shale, chalk, limestone) because of variations in depositional and diageneti processes that affected preservation of biogenic structures. It is suggested that some of the observed differences in these Upper Cretaceous trace-fossil assemblages are more apparent than real. At present, taxonomic problems relating to trace fossils hinder quantification of the Upper Cretaceous assemblages. Direct application of standard taxonomic conventions could lead to conceptual confusion because the morphology of trace fossils is controlled as much by general behavior of organisms and their specific responses to sedimentologic parameters as by body form. For example, Ophiomorpha is known to grade into Thalassinoides, some of which developed Teichichnus-like form. Furthermore, some Rhizocorallium are connected to Thalassinoides burrow systems. The first three genera could be synonymized with Rhizocorallium, which has priority. A more constructive approach involves erection of suprageneric categories, such as Rhizocorallium-group, that would express End_Page 543------------------------------ the natural relationship of many associated trace fossils. End_of_Article - Last_Page 544------------