New Mexico Geological Society Annual Spring Meeting — Abstracts

It has long been agreed that the eastern margin of the San Juan Basin of the Colorado Plateau has undergone some Laramide right shift (about 5-20 km) with respect to the Nacimiento uplift of the Rocky Mountain foreland. Estimates that during Laramide time (Late Cretaceous-Paleogene) the Colorado Plateau moved 60 to 170 km to the north/northeast relative to the craton to the east are based on: (1) estimating the presumed crustal shortening across thrust and reverse faults to the north of the Plateau in the Wyoming province; and (2) estimated displacement of Precambrian terranes in north-central New Mexico. Those who advocate such large-scale decoupling and right slip between the Colorado Plateau and craton have concluded that most of the strike-slip motion took place on faults now buried by the sedimentary fill of the late Cenozoic Rio Grande rift (RGR). However, easterly trending, regional piercing lines across the RGR based on Paleozoic and Mesozoic strata allow a maximum, post-Middle Cretaceous right-lateral offset of only 5-20 km along the eastern side of the Colorado Plateau.

In south-central New Mexico, thin Cambrian-Devonian stratigraphic units are truncated northward beneath unconformably overlying upper Paleozoic strata. These truncated edges form regional piercing lines that indicate no significant right slip immediately SE of the Colorado Plateau. In north-central New Mexico, a regional, low-angle unconformity beneath Cretaceous strata truncates Jurassic strata southward. These strata also thin consistently southward to establish isopach lines that are nearly parallel on the eastern and western sides of the RGR. Particularly significant is the wedge-edge of the lower, limestone-dominated Luciano Mesa Member of the Todilto Formation (Middle Jurassic). It, too, establishes a piercing line E-W across the RGR that does not allow 60-170 km of Laramide right slip.

Middle Cretaceous strata in northern New Mexico also establish piercing lines across the RGR based on the trends of depositional pinchouts and facies tracts. The Hosta-Dalton Sandstone, Gallup Sandstone, basal Niobrara-age (Tocito) sandstone bars and the Tres Hermanos Formation establish piercing lines across the RGR that have a consistent NW-SE trend. Restoring 60-170 km of right slip across these piercing lines produces impossible geometries of these well-studied depositional systems.

In New Mexico, regional piercing lines, isopach intervals and facies changes in strata of Cambrian-Middle Cretaceous age are remarkably consistent--they constrain the amount of post-Middle Cretaceous right slip on the eastern side of the Colorado Plateau to a minimal amount of no more than 5-20 km. Estimates of 60-170 km of Laramide right slip are not consistent with these data.