EROSION – BASED PARASEQUENCES AND PARASEQUENCE AMALGAMATION IN MIXED SANDSTONE – DOLOSTONE DEPOSITIONAL SYSTEMS, LOWER GRAYBURG FORMATION (PERMIAN), GUADALUPE MOUNTAINS, NEW MEXICO

The lower Grayburg Formation is a mixed dolostone – sandstone succession that onlaps the San Andres Formation in the northern Guadalupe Mountains of New Mexico. This study focuses on the details of the onlapping Grayburg succession at Last Chance Canyon, specifically interpreting depositional environments for the carbonate and siliciclastic lithofacies, describing parasequence architectures, and mapping the lateral variations in facies and stratal architecture approaching the San Andres highstand margin.

Sandstones are generally well sorted and range from medium to fine grained feldspathic quartz arenites. Four major lithofacies are recognized: 1) trough cross – stratified, 2) symmetrical ripple laminated, 3) planar laminated, 4) crinkly laminated and brecciated. The trough and ripple facies often display distinctive marine trace fossils. Carbonate lithoclasts are observed in the stratified facies while clotted dolomite matrix is recognized in the crinkly laminated facies. Dolostone lithofacies include homogeneous dolomudstones and laminated dolomudstones to dolopackstones with trace silicate sand.

These lithofacies are organized into shallowing – upward packages here interpreted as parasequences. An ideal succession is floored by an erosion surface overlain by subtidal, tide/current - dominated bars overlain by wave – dominated sand flats that are in turn overlain by intertidal sand flats with crinkly stromatolitic fabrics. Silicate sand facies are conformably overlain by the mud – dominated carbonates that are here interpreted as supratidal ponds and mudflats. It is here interpreted that the silicate sands are a palimpsest, delivered to the platform through eolian processes during the terminal phase of parasequence progradation and subsequently re-worked in the subtidal realm during parasequence flooding.

Lateral tracing of beds towards the low accommodation San Andres highstand margin demonstrates that carbonates are truncated beneath the basal erosion surfaces of parasequences, resulting in sand-on-sand contacts and amalgamation of parasequences. Thus, onlap of the Grayburg onto the San Andres is a complex of platform flooding and parasequence amalgamation up - dip with fully preserved parasequences over the down – dip falling stage clinoforms.