Curious Proximal Facies: Transgressive Overprinting in the Ferron Sandstone
Paul B. Anderson and Ryan King
The Turonian-Coniacian Ferron Sandstone, near Emery, Utah, is subdivided on the basis of regressive-transgressive deltaic cycles. The fourth parasequence set (Kf-4) of the Ferron Sandstone has an overly thickened landward sandstone. This study provides evidence that this thickened portion of the Kf-4 sandstone, in the Bear Gulch area, consists of a landward stepping, or initial relative sea level rise before the progradational cycle began. Recognition of these landward palimpsest facies has likely been hindered by modification associated with transgressive erosion, and deep biogenic overprinting by subsequent distal shoreface environments. The Kf-4 sand body in the Bear Gulch area can be broken into two very distinctive facies associations that are interpreted as a lower wedge of palimpsest nearshore facies, and the overlying distal shoreface deposits that shallow upward. The proximal palimpsest deposits are composed of a highly complex arrangement of facies showing a wide range of environmental conditions. Sedimentary structures are mostly high-energy structures such as planar tabular or trough cross-beds and low angle dipping planar bedding with parting lineations. Periods of rapid sedimentation are suggested by the presence of equilibrichnia traces. However, the occurrence of multiple dinosaur track horizons and an intra-sand surface with vertical tree casts, indicate exposure, perhaps for significant periods of time. Ophiomorpha and Teredolites still lend to the environment being in proximity to marine conditions. A rapid shift is seen both upward and laterally into heavily bioturbated facies and finer grain size, representing a shift to deeper shoreface conditions. Traces can be highly diverse including Ophiomorpha, Diplocraterion, and Rosselia. The sandstone grades upwards into trough cross-beds and planar beds of the upper shoreface and foreshore. However, in many parts of the study area the upper portions of this shoreface are not present due to subsequent fluvial incision. The complex juxtaposition of these facies provides a cautionary tale to subsurface stratigraphers and reservoir engineers.
AAPG Search and Discovery Article #90169©2013 AAPG Rocky Mountain Section 62nd Annual Meeting, Salt Lake City, Utah, September 22-24, 2013