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Turonian Tidal Inlet Architecture and Ichnological Signature, Upper Ferron Sandstone, Central Utah
(1) Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada.
Outcrop work on the Ferron Sandstone of the Last Chance depocenter in central Utah has largely focused on the architecture of, and fluid flow through, river- and wave-dominated deltaic successions. However, facies attributed to tidal inlet successions of the upper Ferron exhibit much higher permeablities than the lithologies associated with deltaic units. This study integrates core and outcrop data from the Muddy Creek area.
In the study area, the facies most indicative of tidal inlet deposition are fine-grained, planar-tabular and trough cross-bedded sandstone intervals. Locally, cross-beds may exhibit bimodal current directions. This facies generally shows an up-section increase in Ophiomorpha abundance. Numerous collapse structures and the occurrence of fugichnia suggest rapid deposition and loosely packing grains. These sandstones are interpreted to represent active inlet fill.
Facies interpreted to represent passive fill within abandoned tidal inlets overlie active fill deposits and are dominated by low diversity bioturbated mudstone successions. Coal, roots, iron nodules, and shell debris are present locally supporting the interpretation that these mudstone beds were deposited under stressed or brackish conditions. In the study area, mud-dominated fills can occur juxtaposed sharply against the high permeability active fill successions, creating fluid migration barriers.
The inlet fills exhibit various depths of incision into heavily bioturbated massive sandstone units that are interpreted as middle shoreface deposits. In some cases, the inlet facies may replace the entire coarse clastic succession (i.e., the heavily bioturbated sandstone facies and the underlying hummocky strata). The basal surfaces of these fills exhibit large Skolithos that penetrate into underlying sandstone units and passively-filled Thalassinoides that penetrate into mudstone units. These contacts are interpreted as Glossifungites demarcated discontinuity surfaces. The varying depths of channel incisions resulted in overlapping and composite Glossifungites surfaces that encompass multiple events of inlet incision and periods of subaerial exposure. The occurrence of mud-dominated inlet fills are crucial components of subsurface models in that they provide impervious barriers that restrict lateral and vertical fluid flow in high permeability targets.