New Mexico Geological Society Annual Spring Meeting — Abstracts

Thousands of invertebrate and vertebrate tracks and trails are preserved in the Robledo Mountains of Dona Ana County, southern New Mexico (UTM 3584120N, 323070E, zone 13). These tracks and trails are in ripple-laminar silty sandstone of the Abo Formation in an intertongued Abo-Hueco sequence of presumed Early Permian (Wolfcampian) age. Some of the tracksite bedding planes also contain plant debris of the conifer Walchia, raindrop impressions, mud drapes and/or mudcracks. The stratigraphic sequence (in ascending order) at the tracksite is: (1) 4.0 m of olive-gray Hueco packstone; (2) 1.5 m of light brown, troughcrossbedded Abo sandstone; (3) 6.7 m of grayish red, laminar and ripple-laminar, silty sandstone; the lower 5 m contain the tracksite; (4) 1.3 m of moderate brown silty mudstone with root tubes and calcrete nodules; (5) 1.2 m of olive-gray Hueco packstone; (6) 6.0 m of grayish red intercalated beds of ripplelaminar Abo sandstone, sandy siltstone and silty shale; and (7) 9.1 m of olive-gray Hueco packstone. All sandstones are very fine-grained micaceous litharenites

The stratigraphic sequence at the tracksite most resembles an asymmetrical cycle of deposition as identified by Mack & James (1986, JSP 56: 635-647). The track-bearing sandstone intervals well corresponds to their "ripple-laminated sandstone facies." This facies is interpreted as part of a siliciclastic tidal flat, more specifically an intertidal sandflat subjected to intermittent subaerial exposure, as indicated by the tracks and trails, mudcracks and raindrop impressions. A detailed, mm-by-mm scale examination of the tracksite sandstone interval is needed to relate tidal cyclicity to trackway formation.