GEOMETRY OF DAGGER MOUNTAIN INTRUSION, BIG BEND NATIONAL PARK, TEXAS

Dagger Mountain located in the Northeastern part of the Big Bend National Park ( BBNP), Texas is a structurally complex area, making it a prime area of research for students and professionals alike. Previous work in Dagger Mountain (Poppeliers, in prep) notes that the igneous rocks observed around Dagger Mountain are limited in extent and not voluminous enough to create any significant, map-scale deformation. The Laramide orogeny (~70-50MA in BBNP) along with Basin and Range extension (~31MA) caused major deformation to the area including emplacement of Tertiary(?) mafic intrusions into Cretaceous limestone. My study aims to determine the overall shape and thickness of the igneous intrusion in hopes of constraining the deformation timing and better understanding the emplacement mechanisms involved.

To the east of mile marker 17 off of US Highway 385, there is an apparently folded sill within the Cretaceous Boquillas limestone. If this sill is related to the nearby Dagger Flats intrusions, the age is ~32 MA. Field observations and geologic maps suggest folding of the sill with surrounding Cretaceous country rock. This implies that Laramide deformation in the area occurred much later than has been proposed for BBNP. Using a proton-procession magnetometer, I conducted a magnetic survey over this feature to better understand the geometry of the intrusion. Modeling of the data suggests a shallow flat sill, not a sill folded with the surrounding Cretaceous limestone. This suggests that the intrusion was emplaced after Laramide deformation of the surrounding Cretaceous units. Other models of sill geometry were attempted including a folded sill and possible ring dikes; however these models did not fit the observed data. Magnetic modeling supports the idea that the sill intruded after Laramide deformation of the Cretaceous limestone.