Lithofacies, depositional systems, and depositional models of the Mississippian Barnett Formation in the southern Fort Worth Basin
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The Barnett Formation in the Llano Uplift region of the southern Fort Worth Basin of north-central Texas is an Osagean-Chesterian age siliciclastic mudrock whose deposition was influenced by the structurally stable Llano Uplift, topographic variabilities, and a long-term, second-order sea-level rise. Pervious work has mostly focused on the producing northern portion of the basin. The present study uses a group of 29 cores to: (1) characterize the Barnett lithofacies, (2) define the depositional setting of each lithofacies and develop a coherent depositional model, (3) identify stacking patterns and correlative surfaces, and (4) establish a viable sequence stratigraphic framework for the succession. On the basis of core data, the Barnett strata are interpreted to have been deposited in a basinal setting, below storm-weather wave-base, under predominantly anoxic bottom waters. The analysis of core and thin sections revealed four dominant lithofacies: (1) laminated siliceous mudstone, (2) laminated calcareous siliceous mudstone, (3) skeletal packstone, and (4) phosphatic packstone and grainstone. Facies stacking patterns were correlated using phosphatic packstone facies as regional marker beds. These beds coincide with changes in clay-mineral abundances, revealed by chemostratigraphic data, and their occurrences were used to subdivide the Barnett strata into lower, middle, and upper units. The lower Barnett is characterized by cyclic sedimentation of extrabasinal clays and has the greatest thickness variability related to accumulation of the calcareous siliceous mudstone facies in graben structures. The middle Barnett is characterized by an increase in extrabasinal clay abundance compared to the lower Barnett, and the upper Barnett is characterized by a decrease in the extrabasinal clay abundance compared to the lower and middle Barnett. The phosphatic packstone facies is sourced from the outer shelf/upper slope of the adjacent Chappel Shelf and is interpreted to represent cycle tops within the aggradational stacking pattern that characterized sediment accumulation style during the second-order sea-level rise that occurred throughout Barnett deposition. The findings contribute to the understanding of the stratal architecture and depositional history of the Barnett deep-water mudrocks and are used to refine the lithofacies variability of the Barnett Formation.