The Trace Fossil Zoophycos From the Shallow Water Facies of the Middle Pennsylvanian Sandia Formation, Jemez Mountains, New Mexico

Abstract

Zoophycos is a large, distinctive trace fossil that is found in marine deposits throughout the Phanerozoic, but has rarely been reported from New Mexico. It has been usually interpreted as the deposit-feeding trace of a marine worm. Zoophycos also gives its name to an archetypal ichnofacies characterized by Seilacher in 1967 as being deposited in deep or at least dysaerobic bottom water. Later it was realized that deep water sediments were not consistently associated with Zoophycos in Paleozoic rocks. The large deposit of Zoophycos traces described here from the Sandia Formation at Guadalupe Box was originally mentioned by DuChene in 1974 and initially described by Kues in 2005. Based on lithology and associated fossils, it was deposited in shallow water, but below wave base, the same environment as was reported for the most recent New Mexico report of Zoophycos , from the Middle Pennsylvanian of Sierra County. At Guadalupe Box, dozens of Zoophycos traces occur in a bed of fine-grained sandstone 28 meters above the base of the 60--meter thick Sandia Formation. Approximately 20 centimeters in thickness, the trace-bearing layer is sporadically exposed to the north for at least 300 meters. At the best exposure, approximately 5 square meters of trace-bearing surface is visible. Beds are close to horizontal and 3-5 cm thick, each with numerous, closely packed Zoophycos traces, 15 to 20 centimeters across. The sandstone is fine-grained and contains a high amount of matrix (32-52%) together with monocrystalline quartz grains, minor polycrystalline quartz grains, and rare detrital feldspar grains. A thin bed of fusulinid packstone limestone two meters above the Zoophycos bed contains Fusulinella, indicative of an Atokan age. The Zoophycus bed is near the top of a large exposure of the lower Sandia Formation. This sequence represents a well-developed fining-upward succession that can be divided into three units, based on lithology. The lower unit, nine meters thick and composed mostly of coarse sandstone, is interpreted to be fluvial. The middle unit, 18 meters of interlayered shale and siltstone, was initially deposited on a coastal plain that became inundated as sea level rose. The upper unit, eight meters of intercalated gray calcareous shale, limestone, and sandstone, was deposited below sea level. The lowermost bed of the upper unit, a grained-supported crinoidal limestone, documents the continuation of transgression, and was deposited in a shallow, open marine setting under moderate to high turbulence. Deepening continued as deposition dropped below wave base, producing limestones with a muddy texture and a diverse fossil assemblage, pointing to deposition in a low energy, but shallow marine environment as long as siliciclastic input was absent. During periods of terrigenous input, calcareous shale was deposited. The Zoophycos bed, and the other thin, fine-grained sandstone strata, may represent distal storm layers. Further study is needed to estimate the degree of oxygenation in these unusual Zoophycos -bearing beds.