Rocky Mountain - 54th Annual Meeting (May 7–9, 2002)

Paper No. 0
Presentation Time: 4:30 PM


MORROW, Jared R., Dept. of Earth Sciences, Univ of Northern Colorado, Greeley, CO 80639, SANDBERG, Charles A., Geologist Emeritus, U.S. Geol Survey, Box 25046, MS 939, Federal Center, Denver, CO 80225 and POOLE, Forrest G., Geologist Emeritus, U.S. Geol Survey, Box 25046, MS 973, Federal Center, Denver, CO 80225,

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.