ORIGIN OF UPPER CRETACEOUS (LATEST MAASTRICHTIAN) MASSIVE, BOULDER-COBBLE CONGLOMERATES, POTRERILLOS FORMATION, LA POPA BASIN, NORTHEASTERN MEXICO: INCISED VALLEY FILL OR MEGA-TSUNAMITE?

The Upper Cretaceous Delgado Sandstone Tongue of the Potrerillos Formation is a thin (<70m), pronounced, basin-wide sandstone/siltstone marker unit within the Difunta Group of the La Popa basin, Nuevo Leon, Mexico. The Delgado Sandstone tongue conformably overlies the Middle Siltstone Member and directly underlies the Upper Mudstone Member of the Potrerillos Formation. Although the depositional character of the Delgado Sandstone Tongue varies both laterally and stratigraphically, it is generally characterized by hummocky and trough cross-stratified, oscillation ripple cross-laminated, massive and bioturbated, fine- to medium-grained sandstone interbedded with subordinate mudstone and siltstone. Sedimentary structures, grain size and ichnofacies are consistent with marine lower to upper shoreface deposition.

A unique, 0.25-10m thick, massive, matrix-supported, boulder to cobble, heterolithic conglomerate containing subordinate, ripple cross-laminated sandstone lenses fills both broad (>20m wide, 5m deep) and narrow, deeply incised (<10m wide, 10m deep), valley-form features within the upper 5-15m of the Delgado Sandstone Tongue. Boulder conglomerates (MPS=20-70cm) are preserved exclusively in narrow, deeply scoured valley-forms; conversely, cobble and pebble conglomerates are preserved in broad, shallow valley-forms. Although clast size varies with the depth of valley-forms, the sedimentologic character and heterolithic clast lithology (presence of intra- and extrabasinal clast/grain types) are laterally and stratigraphically persistent.

Two primary hypotheses explaining the origin of conglomerates and the abrupt break in depositional character of the Delgado Sandstone Tongue include:1) deposition by slumping of steep valley walls within incised valleys (as sub-aqueous debris flows) with concomitant reworking of extrabasinal, lowstand sediments during rapid, marine transgression, and 2) deposition by mega-tsunami pulses induced by the K-T Chicxulub impact. Preliminary field observation and petrographic analyses provide evidence for both models of conglomerate deposition.