ABSTRACT

The rocks exposed in Santa Rosa Canyon, Sierra de Santa Rosa, Nuevo Leon, Mexico, range from late Jurassic (Tithonian) to late Cretaceous (Maestrichtian) and are about 2,000 meters in thickness. The Taraises is predominately dark gray to black, well indurated lime wackestones and mudstones of 132 meters in thickness with the upper part of the formation consisting of 31 meters of dark calcareous shale. The shale contains a few thin beds of moderately indurated lime wackestone which bear ammonites of Valanginian age; other megafossils are rare to lacking. Microfossils are common to abundant in about one third of the samples collected. Induration and recrystallization allowed micropaleontologic study by thin section only. The microfauna includes radiolarians, ostracods, echinoderm debris, and unidentified biogenic grains as well as calpionellids and nannoconids, but only the last two are persistently common.

Calpionellid taxa include Amphorellina subacuta Colom, Calpionella alpina Lorenz, Calpionella elliptica Cadisch, Calpionellites darderi (Colom), Calpionellopsis oblonga (Cadisch), Calpionellopsis simplex (Colom), Remaniella cadis-chiana Catalano. Salpingellina levantina Colom, Stenosemellopsis hispanica (Colom), Tintinnopsella carpathica Colom, and Tintinopsella longa (Colom). The distribution of these taxa indicates that the Taraises ranges in age from middle Berriasian to the Hauterivian-Valanginian boundary. No calpionellids were found in the overlying Tamaulipas (Cupido of many workers).

Although abundant in some thin sections, nannoconids were less useful than calpionellids. Nannoconus steinmanni Kamptner appears intermittently throughout the Taraises Formation. It has been reported to range throughout the Berriasian and Valanginian. Other species of nannoconids were tentatively identified but they are similar in form and range to N. steinmanni and their occurrence was plotted with N. steinmanni.