2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 45-2
Presentation Time: 9:00 AM-5:30 PM


FERGUSON, John F., Geosciences Department, University of Texas at Dallas, PO Box 688, Richardson, TX 75080, GEISSMAN, John W., Department of Geosciences, University of Texas at Dallas, 800 W. Campbell Rd, ROC21, Richardson, TX 75080, KOEBERL, Christian, Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, also of the Natural History Museum, Burgring 7, A-1010 Vienna, Austria, SPILDE, Michael N., Institute of Meteoritics, University of New Mexico, MSC03-2050, Albuquerque, NM 87131 and ALI, Abdul-Mehdi S., Department of Earth and Planetary Sciences, The University of New Mexico, Albuquerque, NM 87131-0001, geissman@utdallas.edu

Decades ago, Mr. John Tackel, an amateur rock collector of Garland, Texas, discovered accumulations of rocks, exposed at Earth’s surface or covered by less than a meter of soil, completely inconsistent with a regional setting of Upper Cretaceous strata, disconformably overlain by Pliocene or younger sediment, and covered by modern soil. He collected tons of material (now at the Department of Geosciences, UTD), with little documentation. His sketch maps of sample locations show an association with near surface exposures of the Austin Chalk (ca. 84 Ma, Santonian) in a narrow NS zone near Duck Creek. These rocks, informally named the “Garland Debris Field” (GDF), are neither sedimentary nor diagenetic in origin. They are dense (~2.8 + g/c3), non-magnetic, of low susceptibility (< 10 x 10-6 SI volume) and non-conductive. A typical fragment is about 20 cm across (some exceed 50 cm). Cut slabs show clear signs of heating and ablation associated with atmospheric re-entry, with fine grained and glassy zones on external surfaces. Thin sections show a dense, well compacted fine-grained groundmass with angular to sub-angular fragments of carbonate rock and rounded to sub-rounded, dark spherules (accretionary lapilli?) up to 8 mm in diameter. Some samples are bowl-shaped, and filled with hundreds of spherules. The rocks are typically coated in a calcareous material. No micro-fossils have been identified. Decades ago JT obtained (undocumented) commercial chemical data of ~ 2.24 ppm Ir, presumably from rocks rich in spherules, and surmised a link to the Chixulub impact event (1500 km distant). At ca. 66 Ma, the Upper Austin Chalk was covered by 100s of m of younger sediments, presenting a quandary regarding the origin of the GDF. New neutron activation analysis data show sub ppb Ir levels (below detection limits) and low siderophile element abundances. Electron microprobe scans show high concentrations of Mn and related elements in the spherules, and small (<50 micron) fragments of Kspar in the matrix, currently being analyzed via 40Ar/39Ar laser fusion. At present, we are not able to link the GDF with an explosive origin from a specific impact or a (highly unlikely) volcanic event.