Foraminifera and testate amoebae morphogroup analysis and biofacies of the Upper Cretaceous (Turonian) Ferron-Notom Delta, Utah, USA

Abstract

Morphogroup and biofacies analyses were conducted on strata of the Upper Cretaceous (Turonian) Ferron-Notom delta, Central Utah, USA. This study investigates associations between ancient deltaic environments and foraminiferal and testate amoebae assemblages with an assessment of their ecological zonation based on their extant Holocene counterparts. Sixty-six hand samples of mudstone were collected from the three outcrops (Caineville North, Blue Hills, and Steamboat) along strike and dip profiles. Morphological analysis showed four morphogroups with eleven morphotypes of foraminifera and testate amoebae reflecting relationships between test morphology and depositional environment. Morphotypes were used to define the main biofacies of the study sections using cluster analysis. Due to a possible fossilization biases with the calcareous foraminiferal fraction, the biodiversity index is only used as relative measure in defining biofacies. Despite this possible taphonomic bias, the agglutinated foraminifera and testate amoebae morphogroups show plausible trends of salinity both among the outcrops and stratigraphically within each outcrop. The Blue Hills outcrop represents the most landward and thus the lowest salinity environment (tidally-influenced backwater), Steamboat is more coast proximal with a slightly higher salinity found in the delta front and the fluvial/estuarine environment, while the Caineville North outcrop represents the most coast proximal with the highest salinity of the prodelta/delta front and fluvial/estuarine environment. It appears that landward and coastward transport of tests led to a taphonomic bias. Nonetheless, the assemblages provide useful information that correlates with previously documented, lithofacies data. A salinity index, which is a ratio of trochospiral taxa versus testate amoebae taxa, provides a useful measure of coastal proximity that matches lithofacies trends. In addition, an Organic Matter (OM) index, which is the ratio of bolivinid-type taxa versus trochospiral taxa shows relationships with original probable OM content of the sediment inferred from the lithofacies and the plant macrofossils.