CRATER STRATIGRAPHY AND CARBONATE IMPACT SIGNATURES, LATE DEVONIAN ALAMO EVENT, NEVADA
Approximately 300 m below the Breccia the late Early or early Middle Devonian 10-m-thick upper member of the Oxyoke Canyon Sandstone contains shocked quartz grains and fluidized pockets that fed individual grains and ductile sandstone clasts into lithic breccia dikes and sills that pinch, swell, shear and fold across the detached top of the Member. The dikes penetrate upward through ~290 m of Middle Devonian platform dolostone that is damaged by pervasive shattercones and sediment- and cement-filled vugs that represent impact shear and partial fluidization.
The overlying Alamo Breccia limestone exhibits three discreet impact facies. The lower ~65 m contains two facies that accumulated quickly during the early stages of the Alamo Event. (1) Heterolithic carbonate clasts 10s of meters in longest dimension are folded, sheared and interpenetrated, interpreted as crater-floor and fallback breccia and probable partial melt. They are overlain by (2) clasts generally less than 10 cm in longest dimension, strongly sheared and attenuated, and destroyed internal bedding, interpreted as partial melt.
Facies (3) is ~65 m of classic Alamo Breccia: clean-washed, clast-supported, thick graded beds, interpreted as tsunami backwash and/or resurge deposits. The fine-grained top of the Breccia merges with 100+ m of thin bedded, slumped, anoxic limestones and rare quartz sandstones, interpreted as post-Event crater fill.
The polymict dikes with clastic matrix contain clasts from the host Oxyoke Canyon and platform dolostone, and perhaps from overlying and/or underlying formations penetrated or mobilized by the impact. Monomict dikes formed locally in both the Oxyoke and the dolostone. The polymict dikes adjusted long enough to imbibe a variety of clasts and matrix, and connect from the Oxyoke Canyon, through the dolostones, and into the lower two facies of the accumulating Alamo Breccia. Based on preliminary mapping, the dikes may represent a listric fault system verging into the crater, and may locally penetrate deeper than the Oxyoke Canyon Sandstone.