DEEP-WATER DEPOSITS OF LATE DEVONIAN ALAMO IMPACT BRECCIA, NORTHERN REVEILLE AND SOUTHERN HOT CREEK RANGES, NEVADA

The offshore early Late Devonian Alamo Impact Event is evidenced in lower-slope settings by radially oriented, megabreccia-filled channels, which are correlated to carbonate-platform Alamo Breccia deposits by conodont biostratigraphy, unusual rock fabric, heterolithic clast composition, and shocked-quartz grains. Lower-slope Alamo Breccia studied in the northern Reveille and southern Hot Creek Ranges locally is >40 m thick and comprises a complex internal stratigraphy characterized by three or more fining-upward units. The basal unit consists of coarse megabreccia containing allochthonous, platform-derived stromatoporoid and carbonate clasts up to 50 cm across, emplaced together with upper- and lower-slope-derived debris and rare, possibly western-derived exotic clasts. Basal unit clast-imbrication paleocurrent measurements indicate transport direction was generally to the S and SE. The middle and upper units are characterized by relatively small, heterolithic clasts, with scattered large, tabular, slope-derived cobbles up to 25 cm long in the uppermost 1–2 m. Preliminary measurements within these units indicate a possible SW, off-platform paleocurrent.

Alamo Breccia sharply overlies and crosscuts rocks as old as Middle Devonian at all five studied lower-slope localities. The disconformity represents a ~3-m.y. time gap and removal of ~10 m of lower-slope section. Locally, breccia-filled dikes penetrate subjacent Middle Devonian rocks to a depth of >5 m. A ~2-m.y. depositional break separates the Alamo from an overlying basal Woodruff Formation debris-flow bed at two Hot Creek Range localities. The basal Woodruff and sub-Event disconformity have been precisely dated at a Hot Creek Range locality near Warm Springs, although the Breccia is absent there. These data suggest an offshore event chronology of widespread, immediate post-impact submarine scour and channeling followed by localized, multi-stage emplacement of Breccia from possibly craterward- and basinward-directed megacurrents associated with transient crater modification.