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Controls On Sedimentation and Cyclicity of the Boquillas and Equivalent Eagle Ford Formation from Detailed Outcrop Studies of Western and Central Texas, U.S.A.
Facies of the Boquillas and equivalent Eagle Ford Formation can be assigned to two associations: a hydrodynamically recycled association and a pelagic association. The hydrodynamically recycled association is composed of lenticular to continuous cross-laminated lime packstone–grainstone, heterolithic facies, and intraclastic grainstone conglomerate with intercalated calcareous mudrock. Sharp bedding contacts, abundant scours, ripple cross-lamination, oscillatory-current indicators, hummocky cross-bedding, and rounded clasts indicate recycling of the bottom sediments by storms and bottom currents above storm wave base (SWB). The pelagic association is composed of continuous lime packstone to grainstone and chalk beds with intercalated calcareous mudrock. Gradational contacts and planar lamination indicate dominant deposition by pelagic rain, but scour surfaces, coarse-fine alternations, and ripple cross-laminae indicate the influence of bottom currents below SWB.
Vertical shifts between the hydrodynamically recycled association, interpreted to represent storm agitation of the seafloor during falls in sea level, alternate with the pelagic association and define four sequence boundaries and three depositional sequences with a lower-order periodicity (third-order periodicity, ca. average 2.4 My). Sequence boundaries correspond approximately to member boundaries recognized by gamma-ray profiles. Spectral analysis reveals higher-frequency mudrock–carbonate cycles prominent in the pelagic association, likely resulting from Milankovitch climate forcing of planktonic carbonate productivity versus clay flux. Comparison of gamma-corrected spectra with absolute age constraints reveals that the Ernst Tinaja section (in Big Bend National Park) is less than 28% complete. This is interpreted to result from abundant depositional hiatuses and erosional gaps that occur at the bed scale and at member boundaries within and bounding the Eagle Ford Formation.
We demonstrate that deep-marine pelagic deposits in epicratonic settings may be subject to a complex interaction of depositional processes including pelagic rain and recycling by bottom currents and wave-generated currents modulated by low-frequency sea-level fluctuation and high-frequency fluctuations in sediment supply.
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