TRANSECT THROUGH POST-IMPACT SEDIMENTS DEPOSITED IN THE OUTER RIM AND ANNULAR TROUGH OF THE ALAMO IMPACT CRATER, NEVADA

The marine-target Alamo impact is preserved by the Late Devonian Guilmette Formation in dozens of ranges of southeastern Nevada, and ongoing research is characterizing the geometry of the crater. Field mapping in the Golden Gate, Monte Mountain, and Pahranagat Ranges document post-impact limestone and sandstone units filling the Alamo crater. Detailed facies analysis demonstrates a shallow-to-deep-to-shallow pattern across the ~50 km N-S transect. Shallow-water facies in the Golden Gate, northern Monte Mountain, and southern Pahranagat sections include open marine fossil assemblages dominated by large stromatoporoids, corals, brachiopods, and crinoids, and occasional syndepositional dolomite. Between these sections, deeper-water facies in the southern Monte Mountain and northern Pahranagat localities are characterized by thin-bedded barren mudstones, intervals of pelagic fossils, and hiatal firm- and hardground surfaces. Shallow-water facies tend to be limestone-dominated with thin sandstone units, whereas deeper-water facies show the inverse pattern. The main depocenter of the quartz sand occurs in the central part of the transect, which may help define the boundary between the annular trough and outer rim regions of the impact crater. Field relationships suggest a conservative diameter of 29 km for the annular trough, which is located east of rim deposits at Tempiute Mountain.