Post-Orogenic Exhumation of the Northern Appalachian Basin and the Timing of Hydrocarbon Generation: Low-Temperature Thermochronological Constraints From New York and Pennsylvania

Abstract

The Northern Appalachian Basin, located in New York (NY) and Pennsylvania (PA), contains a well-preserved sedimentary record and significant unconventional gas reserves. This study applies apatite fission-track thermochronology and (U-Th)/He dating to constrain the post-orogenic thermal history and exhumation of the Northern Appalachian Basin. Apatite fission-track (AFT) thermochronology is particularly useful since the system contains a kinetic parameter (i.e. track length distributions) and is sensitive to nearly the same temperature range (~50-130 °C) over which hydrocarbons are generated on geological time scales. AFT data (i.e. ages, track lengths, composition proxies) were collected from Devonian sandstones along an east-west transect straddling the NY-PA border. (U-Th)/He dating was conducted on select samples. Inverse thermal models constrain best-fit time-temperature envelopes from thermochronologic data, combined with existing stratigraphic and maximum paleo-temperature constraints (i.e. conodont alteration index and vitrinite reflectance). AFT ages range from ~185-120 Ma with mean track lengths of 10.6-13 µm, with unimodal, slightly negatively skewed distributions. With exception of the western most samples, all samples reached temperatures higher than the retentivity of fission tracks in apatite (i.e., >~110-120 °C) and thermal maxima was reached after the Alleghenian Orogeny (mid-Pennsylvanian). Inverse thermal modeling shows several trends: (1) In the Northern Appalachian Basin, cooling from maximum temperatures and the beginning of optimal hydrocarbon generation conditions began in the Early Jurassic. (2) Western and west-central samples reached lower maximum temperatures and constrain slower cooling rates with longer residence times within the optimal temperature range as compared to the simple, relatively rapid cooling paths of eastern samples. (3) Initial cooling is likely due to erosion of the orogenic overburden, which was initiated at similar times across the basin with greater erosion in the east as reflected in the stratigraphy and AFT ages; (4) After removal of the orogenic overburden, cooling rates slow as the Northern Appalachian Basin stabilizes from the Early Cretaceous to mid-Eocene; (5) The western samples record a rapid cooling event during the mid-Eocene to present which may be due to landscape rejuvenation. This study adds to the thermochronologic understanding of the Eastern United States since the Early Jurassic.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018