Evaluating Laramide Orogenesis Via Flexural Basin Response In The San Juan Basin, New Mexico And Colorado
— Kurt Rudolph, Ryan Leary, Tyson Smith, and Kristine Zellman

A challenge in interpreting the location, timing, and magnitude of ancient orogenic events is that ongoing uplift and erosion
in the hinterlands often destroys much of the primary record of these events. However, basin-thickness patterns in the sedimentary record
can provide complementary evidence of uplift via flexural effects. Here, we deploy well-log correlation, isochores, basin modeling, flexural
modeling, and subcrop mapping to evaluate the Late Cretaceous to Paleogene basin response to Laramide tectonism in the San Juan Basin.
A wedge of upper Campanian to Maastrichtian sedimentary rock thickens from 200 to 800 m from southeast to northwest in the basin. This
pattern can be successfully simulated via flexural modeling if we infer early Laramide uplift along the northwest basin flank that produced
a 0.8-km-high topographic load. The Laramide unconformity bounds the top of this Upper Cretaceous sedimentary wedge and truncates
progressively older strata to the east, further supporting a westward tilt of the basin. The onset of Campanian Laramide flexure may have also
contributed to the profound transgression from the upper Menefee Formation to the Lewis Shale. The Paleocene isochore map displays an
approximately symmetrical pattern, with thickening toward the center of the basin. This suggests the possibility of competing flexural loads.
The base Eocene structure indicates an asymmetric deep on the northeast flank of the basin, providing flexural evidence of contemporaneous
uplift/loading of the Nacimiento uplift and Archuleta arch; this has been modeled as ~2.1-km load height. Both Cretaceous and Paleocene
sedimentary wedges are narrow, suggesting low flexural rigidity; modeled effective elastic thicknesses (EET) are 20–30 km, comparable to
estimates of modern EET for the region.