New Mexico Geological Society Annual Spring Meeting — Abstracts

Jurassic strata exposed along the southward extension of the Picuris-Pecos fault system south of Lamy, Santa Fe County (T12-13N, R10E), are assigned to the San Rafael Group (Entrada, Todilto and Summerville formations) and overlying Morrison Formation (Salt Wash, Brushy Basin and Jackpile members). A complete section of the San Rafael Group just north of Arroyo de La Jara consists of four lithostratigraphic units: (1) Dewey Bridge Member of Entrada Sandstone at the base of the section rests sharply on mudstones at the top of the Upper Triassic Chinle Group, is ~5 m thick and consists of white and pink, fine- to mediumgrained, ripple-laminated and trough-crossbedded sandstone; (2) Slick Rock Member of Entrada Sandstone is ~18 m thick and is yellow and gray, fine- to medium-grained sandstone with large trough crossbeds or tabular beds with climbing ripples; (3) Luciano Mesa Member of Todilto Formation, ~4 m thick, begins with 0.3 m of waterworked fine sandstone followed by 2.8 m of dark gray, finely laminated, kerogenic limestone capped by 0.7 m of brecciated and vuggy limestone; and (4) the Summerville Formation, ~19 m thick, is pale brown, cyclically bedded, very fine-grained sandstone and siltstone with some gypsum (6 m thick) overlain by interbedded gray limestone (with red chalcedony nodules) and reddish brown mudstone (~3 m thick) capped by ~10 m of reddish brown mudstone with thin, lenticular beds of trough-crossbedded sandstone. The base of the Morrison Formation on the Summerville Formation is a sharp, erosionally scoured surface overlain by medium-grained and pebbly sandstone at the base of the Salt Wash Member. This section of the San Rafael Group is significant to regional San Rafael Group stratigraphy in several ways: (1) it is the easternmost known section of the Dewey Bridge Member and thus the easternmost sedimentary record of the Carmel transgression; (2) it is east of the Todilto gypsum (Tonque Arroyo Member) pinchout and thus another zero isopach point for that unit; (3) it further demonstrates the continuity of the Summerville Formation across northern New Mexico; and (4) its sharp contact with the overlying Salt Wash Member of the Morrison Formation is the most compelling local physical stratigraphic evidence of the J-5 unconformity.