ICHNOFABRICS AND GEOMETRIC CONFIGURATIONS OF OPHIOMORPHA WITHIN A SEQUENCE STRATIGRAPHIC FRAMEWORK - AN EXAMPLE FROM THE UPPER CRETACEOUS US WESTERN INTERIOR

Ichnofabric in the Upper Cretaceous Sego Sandstone and Anchor Mine Ton gue of east-central Utah can be differentiated into two components: (1 ) discrete Ophiomorpha and (2) mottling and other trace fossils. The i chnofabric index method was employed to evaluate spatial variability o f the ichnofabrics within depositional sequences and component systems tracts. Indices were logged for amount of bioturbation caused solely by Ophiomorpha (Oii) and that represented by all other biogenic featur es (Bii). Values of Oii > 1 are more pervasive in lowstand systems tra cts compared to transgressive systems tracts. This is consistent with the predominance of marginal and nearshore marine, sand-dominated sett ings that are characteristic of lowstands, which are favourable habita ts for colonization by Ophiomorpha producers. Ichnofabric index values vary both vertically and laterally within any given systems tract, re flecting differences in physical and biological parameters operating i n the palaeoenvironment. These parameters include the total number and behaviours of organisms occupying the substrate, as well as substrate texture and grain size, and rates of sedimentation. The architectural style of Ophiomorpha was examined within five depositional facies: sh elf, storm deposit, lower shoreface, shoreface, and estuarine. Inclina tions of individual burrow elements were approximated relative to bedd ing planes, categorized as either vertical, inclined or horizontal, an d then plotted on ternary diagrams. Based on the types of facies prese nt, these results suggest that variations in the geometric configurati ons of Ophiomorpha are controlled primarily by physical energy levels, and the rate and nature of sedimentation. Results of this study have broad implications for understanding the physical factors affecting fa cies variability within sequences and systems tracts. When coupled wit h sedimentologic data, recognition of variations in the distribution o f ichnofabrics and architectural style of Ophiomorpha can provide addi tional information useful for characterizing depositional environments , and therefore could be integrated with other basin analysis techniqu es to test and refine sequence stratigraphic interpretations.