We report detrital zircon U-Pb ages in the Fort Worth Basin (southern USA) aimed at understanding sediment dispersal patterns on the southern margin of Laurentia before and during the Laurentia-Gondwana collision. The ages from two Cambrian fluvial-marginal marine sandstone and six Pennsylvanian deltaic-fluvial sandstone samples span from Archean to early Paleozoic time. In the Cambrian sandstones, 80% of zircons are of Mesoproterozoic age (1.451-1.325 Ga) and 18% are of Grenvillian age. The high abundance of the Mesoprotero zoic population suggests that the grains were dispersed by a local river draining the midcontinent granite-rhyolite province located in the Texas Arch to the northwest of the Fort Worth Basin. In the Pennsylvanian sandstones, 26% of zircons are of Archean-early Mesoproterozoic age, 47% are of Grenvillian age, 15% are of Neoproterozoic-earliest Paleozoic age (800-500 Ma), and 10% are of early Paleozoic age (500-318 Ma), indicating a different dispersal pattern during the Pennsylvanian relative to the Cambrian. Compared to other early Paleozoic detrital zircon records on the southern margin of Laurentia, our Pennsylvanian sandstones have a distinct age peak at ca. 650-550 Ma, which we interpreted to be a result of transport by local rivers draining a peri-Gondwana terrane, most likely the Sabine terrane in the Ouachita orogen. The high abundance of Grenvillian zircons reflects either direct transport from the Appa lachians by an axial river or recycling from Mississippian-Pennsylvanian sedimentary rocks incorporated in the Ouachita orogenic front. The similarity of detrital zircon age distributions in the Fort Worth Basin, the Arkoma Basin, and the southern Appalachian forelands seems to favor sediment dispersal by a major river with headwaters in the southern Appalachians.
Small-volume Late Cretaceous monogenetic alkaline volcanism along the southern margin of North America resulted in a broad igneous belt more than 1200 km long from the Trans Pecos region of west Texas to central Mississippi, collectively forming a northern Gulf of Mexico magmatic zone (NGMMZ). The locus of igneous activity is associated with the discontinuity separating Mesoproterozoic cratonic lithosphere and Jurassic transitional lithosphere, a zone approximating the southern margin of Laurentia, the subsurface trend of the Pennsylvanian Ouachita orogenic belt, and the trace of the Miocene Balcones fault zone in Texas. Although previous studies have attempted to determine the ages of igneous activity in the region, few well-constrained geochronologic data using modern high-resolution techniques are available. We determined the age of eruption for the Balcones igneous province (BIP), a 400-km-long subsegment of the NGMMZ, using modern 40 Ar/ 39 Ar and U-Pb geochronology methods. Our results suggest that previously reported 40 K/ 40 Ar and 40 Ar/ 39 Ar ages underestimate the age of igneous activity by as much as 17%. New ages from this study, along with reevaluation of previous results, suggest that BIP igneous activity occurred in two discrete phases, each lasting only 2.6 m.yr. and separated by 2.7 m.yr.: older mafic volcanism occurred between 81.5 and 84.1 Ma and younger felsic volcanism between 76.2 and 78.8 Ma. The total interval of 8 m.yr. for BIP igneous activity is much shorter than had previously been inferred. The best geochronologic results are obtained from U-Pb dating of zircon mineral (phonolites) separates and 40 Ar/ 39 Ar dating of phlogopite (nephelinites and basanites) and amphibole (phonolites) separates.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.