The Upper Cretaceous Beartooth Sandstone of Southwestern New Mexico: A Transgressive Deltaic Complex on Silicified Paleokarst

Abstract

The Beartooth Sandstone in the Silver City Range, southwestern New Mexico, is part of a long, linear belt of transgressive, paralic deposits of Late Cretaceous age. The Beartooth rests unconformably on an extensive paleokarst surface developed on Paleozoic-age strata. Paleo-caves, pipes, and other karst features are filled with silicified detritus on the order of 30 m thick and, in places, overlain by an in situ terra rossa soil. Silica for the chertification of the karst-fill was provided by the thorough weathering of the terra rossa soil. The Beartooth Sandstone itself represents a general transgressive deltaic deposit complicated by a minor regressive phase and several shifts in the sites of deltation. The initial transgressive deposits (12 m thick) represent a low energy reworking of the soils and karst-fill in a strandline to shallow nearshore zone laterally associated with deltaic deposits. These strata are muddy, very fine sandstones to siltstones with an abundance of wood chip and tree limb impressions, no skeletal material, and a paucity of burrows. These lowermost Beartooth Sandstone strata are vertically succeeded by a massive, unipolar, thoroughly trough cross-stratified medium sandstone (4 m thick) that was deposited as part of a distributary mouth bar. Overlying the lower distributary mouth bar deposit is a ripple cross-laminated and burrowed siltstone in which occur thin cross-stratified and parallel laminated beds of sandstone displaying ball and pillow structures. These strata represent deposition in a prodelta environment. The 5-m-thick prodelta sequence is succeeded by another unipolar, thoroughly trough cross-stratified sandstone containing conglomerate lenses. This upper distributary mouth bar accumulation represents either a regression and/or progradation as a result of deposition concomitant with a stillstand of sea level. Capping the sequence is the thick accumulation of predominantly shales and siltstones of the Colorado Shale. The lower part of these strata accumulated as part of the prodelta and were succeeded vertically by normal open marine shelf deposits.