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 Tongue of east-central Utah can be differentiated into two components: (1) discrete Ophiomorpha and (2) mottling and other trace fossils. The ichnofabric index method was employed to evaluate spatial variability of 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 features (Bii). Values of Oii> 1 are more pervasive in lowstand systems tracts compared to transgressive systems tracts. This is consistent with the predominance of marginal and nearshore marine, sand-dominated settings that are characteristic of lowstands, which are favourable habitats for colonization by Ophiomorpha producers. Ichnofabric index values vary both vertically and laterally within any given systems tract, reflecting differences in physical and biological parameters operating in 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: shelf, storm deposit, lower shoreface, shoreface, and estuarine. Inclinations of individual burrow elements were approximated relative to bedding planes, categorized as either vertical, inclined or horizontal, and then plotted on ternary diagrams. Based on the types of facies present, these results suggest that variations in the geometric configurations 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 facies variability within sequences and systems tracts. When coupled with sedimentologic data, recognition of variations in the distribution of ichnofabrics and architectural style of Ophiomorpha can provide additional information useful for characterizing depositional environments, and therefore could be integrated with other basin analysis techniques to test and refine sequence stratigraphic interpretations.