Origin of an unusual limestone breccia bed in the Boquillas Formation (Upper Cretaceous), Big Bend National Park, Texas
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The Boquillas breccia bed exposed at Ernst Tinaja in Big Bend National Park is a unique bed within the Boquillas Formation, and must record a unique event that occurred during the c. 25 million year depositional history of the formation. The breccia bed is about 10 cm thick, and consists of semi-consolidated Boquillas limestone rip-up clasts supported in a matrix of sandy skeletal phosphate grains and inoceramid shell fragments. Several thin layers of calcareous shale interrupt the breccia bed, and divide it into three or four depositional units. The features of the breccia bed suggest that it represents the deposit of a cohesionless debris flow, initiated by disturbance and downslope movement of a mass of semi-consolidated Boquillas sediment. Mixing of the material with ambient seawater allowed for disintegration of the originally bedded sediment into flat pebbles, segregation of the coarse fraction of skeletal phosphate and shell fragments into bedload, and suspension of the fine fraction into the water column above the flow. The debris must have accumulated in several surges, separated by quiescent periods of several hours or days that allowed for the fine draping layers to accumulate. The mechanisms that may trigger deposition of these sort of beds has been a subject of widespread interest. Surface waves generated by severe storms or tsunami may have sufficient wavelength to set the bottom sediment in motion even in very deep water, however, this should leave evidence in the form of oscillatory ripple cross-lamination or hummocky cross-stratification indicative of traction transport at the sediment surface. Similarly, storm surge or ebb flow or tsunami-generated run-up or ebb flow would be expected to produce structures indicative of unidirectional flow at the bed surface. Cyclic loading of the bottom sediment during passage of large surface waves or shaking due to transmission of seismic waves would be expected to result in soft sediment deformation, liquefaction, or fluidization. None of these features generally thought to be especially diagnostic of tempestites, tsunamiites, or seismites are observed in the Boquillas breccia bed, although the bed exhibits some features compatible with all three of these. Similarly, breccias generated by meteor impacts (impactites) should contain fragments ejected from the target rocks, quenched melt material, and shock-metamorphosed or high-pressure mineral grains. However, debris flows initiated at great distance from the impact site typically contain very little of this material, and it may be present only in minute amounts in the atmospheric fall-out that follows such an impact event. Petrographic study of the breccia clasts and matrix revealed no particularly unusual features, or minerals not found elsewhere in Boquillas limestones. The precipitation of interstitial pyrite and kaolinite within the shells of foraminifera and within the breccia matrix, as well as its subsequent oxidation to hematite must have occurred shortly after deposition because many of the reworked clasts in the breccia show evidence for oxidation while others do not. This could be due to initial deposition of the sediment under reducing conditions and later exposure to oxidizing conditions prior to disruption to form the breccia. Precipitation of sparry calcite cement later took place within and between the particles throughout the breccia. The markedly negative oxygen isotope values for the breccia may indicate that the calcite spar cement was precipitated when fresh water was flushed through the rock during later uplift.