Evolution of Laborcita Formation fan-deltas, Sacramento Mountains, New Mexico

Fan-deltaic sequences in the Laborcita Formation, northern Sacramento Mountains, New Mexico, were an important component of a northwesterly prograding late Paleozoic clastic wedge. These Pennsylvanian/Permian (Virgilian-Wolfcampian) deposits represent a facies tract of increasing maturity and sorting to the northwest. Redbed conglomerates interpreted as braided and ephemeral fluvial channel-fills crop out at the most southern extent of the study area (Highway 82), and are the most proximal deposits. These limestone cobble conglomerates are interpreted to be syntectonic deposits due to their proximity to the Fresnal Canyon Fault. Sheet-like fanglomerates at Dry Canyon are in erosional contact with marine limestones and correlate with the highway 82 deposits. The upper surfaces of these fanglomerates were winnowed and reworked by marine processes. A fan-delta outcrop below the Tertiary sill in Laborcita Canyon contains large scale foresets with conglomerate-filled channels which connect fluvial topsets and bottomset conglomerates. The basal conglomerate is interpreted to be a frontal splay deposited during major flood events. A second fan-delta outcrop in upper Laborcita Canyon contains a higher percentage of sand and small scale more continuously deposited frontal splays. These thin (0.5 m) sand lenses closely resemble suprafan deposits on submarine fans. Sands deposited on the fringe of this fan-delta contain complete and partial, proximal to distal sequences of upper flat-bed, climbing-ripples, and burrowed lower flat-bed. Sand-rich fan-delta deposits at Tularosa Canyon are composed of two foreset units and an upper topset channel. The lowest unit has hummocky cross-stratified toesets that evidence marine reworking. The upper foreset-unit contains a channel surrounded by syndepositionally deformed interbedded sandstones and shales. The entire sequence is capped by shales containing caliche nodules and a limestone deposited on the abandoned and foundered delta platform. Quartzite cobble conglomerates at Bookout Lane and above Laborcita Canyon are thick (10 m) and laterally extensive (6 km). These conglomerates are poorly cemented and contain trough cross-bedded coarse sandstone lenses associated with intrachannel bars. These conglomerates are interpreted to be similar in composition and morphology to the midfan deposits which fed the conglomerate-free fan-deltas at Tularosa Canyon and Hightower's Cliff. Thick (10 m) Sands at Hightower's Cliff contain toesets which were deposited by slumping of oversteepened foresets. These fan-delta fringe deposits resulted in geometries simialar to submarine fan suprafans. This fan-delta is overlain by a phylloid algal mound. Fan-delta positive relief and subsidence due to shale loading is interpreted to have influenced mound growth. A fan-delta at Coyote Canyon is sand-poor and contains a rhyolite cobble conglomerate channel. This deposit is relatively thin, and contains a limestone with syndepositional structures. This fan-delta is interpreted to have rapidly prograded across a shallow lime mud shelf. Fan-delta source areas shifted during Laborcita deposition. Initial deposits were from the southeast and characterized by conglomerates with limestone and chert clasts. Later quartzite/rhyolite porphyry cobble conglomerates prograded from the east and northeast. Increasing water depth and sand content of fan-deltas from Dry Canyon through Hightower's Cliff caused the observed changes in morphology. The fan-delta at Coyote Canyon represents the final fan-delta progradation of igneous basement derived clasts across a shallow marine shelf. Alternations of progradational and transgressive Laborcita facies are due to autocyclic shifting of depocenters, localized tectonism and gradual subsidence. Ultimately, alluvial fan redbeds of the Abo Formation prograded terrestrial facies over Laborcita fan-deltas and marginal marine facies