ACTIVE TECTONISM DURING CARBONIFEROUS-PERMIAN TRANSITION, CENTRAL NEW MEXICO

An active phase of the Ancestral Rocky Mountains orogeny took place during deposition of the Bursum Formation, which is transitional between largely marine Upper Pennsylvanian strata and terrestrial redbeds of the Permian Abo Formation. Angular unconformities, angular off-lap of strata, faults that die out upward, clastic dikes and other earthquake-liquefaction features indicate tectonism during Bursum deposition. Markedly different facies of the Bursum are closely juxtaposed across faults northeast of Socorro. On upthrown blocks, the formation is less than 30 m thick and comprises alternating marine limestone, conglomerate, and variegated claystone paleosols. Conglomerates contain pebbles of quartz and fresh pink feldspar derived from nearby Precambrian outcrops. On downthrown blocks the Bursum thickens to 100 m of largely greenish to olive-gray, laminated mudstone and siltstone, commonly containing fossil land plants and rare insect parts. Carbonate-filled clastic dikes occur close to faults that displace the Bursum, but not younger Abo Formation. At Carrizo Arroyo in northwest Valencia County, the same facies yields an exceptionally diverse marine and non-marine fossil assemblage. Also at Carrizo Arroyo, a large sandstone dike terminates into a preserved surficial sand blow.