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Comparative Analysis of Internal Characteristics of Delta Front Deposits of the Loyd Sandstone (Campanian) Near Rangely, Colorado, Panther Tongue of the Star Point Formation, Helper, Utah, and Lower Sego Near Floy Canyon: Controls Versus Reservoir Properties


Integrated sedimentologic, ichnologic, and architectural analyses indicate that the Loyd Sandstone near Rangely, CO comprises several multimeter-thick, heterolithic, coarsening upward successions interpreted as mixed fluvial-flood and tidally modified delta lobes. Delta clinoforms comprise planar-laminated sandstone interbedded with flaser-wavy-lenticular bedded siltstone, and subaqueous terminal distributary channels that incise into clinoforms. Mud balls, drapes, and rip-up clasts are common. The trace-fossil assemblage is high-abundance, high-diversity, and exhibits shallow to deep tiering. The ichnotaxa--16 fully marine traces; 10 marine, brackish, or freshwater traces; and no exclusively freshwater traces--record salinity tolerances, and help reconstruct paleoenvironments and sedimentation patterns. Vertical Ophiomorpha 2-4 m deep penetrate multiple bedsets, improving overall connectivity. Finer grained intervals between sandy foresets are heavily bioturbated, increasing isotropy and permeability in these beds.

The Loyd delta is related to a high sediment influx during rising sea level. Although volumetrically dominated by sandstones deposited by recurring river floods, interbedded siltstones, mud-drapes, and trace fossils indicate that abundant time is recorded by reduced discharge, finer grained sedimentation, tidal reworking, and intense bioturbation by biotic communities tolerant of mesohaline to polyhaline conditions. Controls on the internal characteristics of the Loyd include: 1) a high sediment influx into relatively shallow water during rising sea level; 2) recurring fluvial floods that reduced salinities, deposited sand, and promoted delta-front channelization; and 3) tidal reworking and bioturbation of sediments during reduced discharge and higher salinities.

Integrated sedimentology, ichnology and drone photogrammetry show that the Loyd differs markedly from the river-dominated delta deposits of the Panther Tongue of the Star Point Formation (the Loyd contains more mud and increased bioturbation) and tidal deltas of the lower Sego Sandstone (the Sego contains more mud with less internal connectivity between sandstones). When using the Loyd delta or any delta as a reservoir analog, it is critical to examine the complex internal stratigraphic and ichnologic characteristics, and not simply consider the overall stratal stacking pattern or the volumetrically dominant facies to deduce the spatio-temporal distribution of controls on deposition and resultant architectures.