Joint South-Central and North-Central Sections, both conducting their 41st Annual Meeting (11–13 April 2007)

Paper No. 4
Presentation Time: 9:20 AM


SANDBERG, Charles A., U.S. Geol. Survey, Box 25046, MS 939, Federal Center, Denver, CO 80225, MORROW, Jared R., Geological Sciences, San Diego State University, 5500 Campanile Dr., 237 GMCS, San Diego, CA 92182 and HARRIS, Anita G., 1523 East Hillsboro Boulevard #1023, Deerfield Beach, FL 33441,

Conodonts ejected by the Late Devonian (early Frasnian, punctata Zone, ~382 Ma) Alamo marine impact evidence derivation within a >1.7-km-deep blast crater with a final diameter of 44–65 km. Generally unshattered conodonts contained within small clasts and redeposited in a single fallout lapillistone block, subsequently reworked within the Alamo Breccia by the post-impact megatsunami, were derived from six formations: lower Frasnian part of Guilmette Formation, Lower Silurian part of Laketown Dolostone, Upper Ordovician Ely Springs Dolostone, Middle Ordovician Copenhagen Formation and Antelope Valley Limestone, and Upper Cambrian part of Goodwin or Hales Limestone. Conodont preservation is fortuitously related to clast size as well as to composition, induration, and conodont abundance of the provenance layer and its depth within the crater. Older conodonts have lower CAI values (1–2) than younger ones (3.5–4), indicating that the heat of impact dissipated through formations with depth. The youngest conodonts, which lay on or in unconsolidated sediments just below the seafloor, commonly are broken and poorly preserved. Unencased conodonts or those contained within tiny fragments probably were totally shattered and are unrecoverable. Other well-preserved Ordovician to Frasnian conodonts occur in lithoclasts within the graded megatsunami breccia.

Redeposited impact-derived conodonts may be as excellently preserved as those reworked in turbidites and debris flows derived from an advancing forebulge (e.g., Famennian Lower marginifera and Lower expansa Zone conodonts in Lower Mississippian Tripon Pass Limestone, Nevada, and McGowan Creek Formation, Idaho), on an offshore carbonate bank (e.g., Upper Devonian, upper tongue of Fenstermaker Wash Formation, Nevada), or in transgressive deposits (e.g., Ordovician to Devonian conodonts in Kinderhookian Bachelor Formation and Famennian Saverton Shale, Missouri). Similarly well-preserved conodonts are also encountered in stratigraphic leaks within sinkholes (e.g., Osagean anchoralis-latus Zone conodonts in Frasnian reefs, Germany). On the other hand, indigenous conodonts reworked by wave action in high-energy peritidal settings may be rounded into small sand grains barely recognizable as conodonts (e.g., Pennsylvanian Casper Formation, Wyoming).