--> Relating Modern Analogs and Process-Based Facies Models to Ancient Deposits: A Mixed-Energy Estuary From the Cretaceous Straight Cliffs Formation, Southern Utah

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Relating Modern Analogs and Process-Based Facies Models to Ancient Deposits: A Mixed-Energy Estuary From the Cretaceous Straight Cliffs Formation, Southern Utah

Abstract

Process-based facies models for estuaries and deltas are largely derived from modern analogs, and generally depict end-member energy settings. It is unclear how applicable these models are to interpretations of the rock record, particularly in more complex mixed-energy estuarine, deltaic, and tidal environments. Such ambiguity reflects the difficulty in understanding preservation potential, the close temporal and stratigraphic interplay between end-member systems, and a general knowledge gap for both modern and ancient high-energy, sand-rich tidal settings. Differences in the temporal and physical scales of observation between modern and ancient examples pose a challenge to analog studies and their application to subsurface reservoirs. This research presents a detailed assessment and model for outcrops of a mixed-energy (wave- and tide-dominated) estuary from the Cretaceous Straight Cliffs Formation, southern Utah, with specific modern analog comparisons. Along a 1,200 m-wide, 60 to 120 m-thick section, cm-scale measured sections, petrography, and photos are used to document vertical and lateral facies changes. The estuary consists of three depositional units (DU): (1) a lowermost interval, 20–30 m thick, of tidal bars and marsh deposits composed of carbonaceous shales and coals; (2) a middle interval, 50–80 m thick, with channelized tidally-influenced bayhead delta / tidally-dominated delta deposits; and (3) an uppermost interval, 30–50 m thick, of landward-stepping barrier island strata. A combination of modern analogs show the evolution of the estuary. The system began similar to the North Carolina coast at Cape Hatteras (DU1). As the estuary filled it looked more like Winyah Bay, South Carolina (DU2). The capping barrier island strata could be similar to Big Sarasota Pass, Florida or the East Friesian Islands, Germany (DU3). This study highlights areas for improvement in the modern to ancient to reservoir analog workflow, particularly in mixed-energy systems. For example, we illustrate the difficulties in distinguishing between bayhead and tidal deltas in outcrop, despite the importance of such distinctions for both sequence-stratigraphic and reservoir interpretations. Overall, detailed facies characterization and predictive 3-D geobody analysis does elucidate key recognition criteria for the mixed-energy system, including the preservation of both tide and wave energy indicators, tidal packages, and barrier island facies.