Post‐Alleghanian unroofing history of the Appalachian Basin, Pennsylvania, from apatite fission track analysis and thermal models

Abstract

Results of apatite fission track analyses on 29 Ordovician through Permian sandstones from the Appalachian Basin in Pennsylvania are presented. Ages range from 111±17 to 184±10 Ma. Mean track lengths of 10.71±0.29 to 13.10±0.17 μm with unimodal, negatively skewed length distributions are indicative of slow cooling. The data separate into two groups on an age versus mean length plot. The younger group (111–144 Ma) is found in the structural depressions of the Anthracite Basin and the Broad Top Basin and adjacent Appalachian Plateau. The older group (144–184 Ma) is found in the structurally higher southwestern Appalachian Plateau and Juniata Culmination and adjacent central plateau. Fission track data suggest that the basin cooled slowly after the Alleghanian Orogeny, with culminations cooling earlier than depressions. Cooling histories modeled from apatite fission track data, with maximum temperatures constrained by vitrinite reflectance, indicate cooling beginning soon after the Alleghanian Orogeny except in the Juniata Culmination, which apparently experienced synorogenic cooling and unroofing during formation of the underlying duplex. Model cooling histories and available geologic information indicate that the foreland basin did not experience Mesozoic reheating. Unroofing histories were modeled from fission track cooling histories using heat flow estimates and burial depths derived from vitrinite reflectance profiles. The models suggest that the unroofing history of the Appalachian Basin in Pennsylvania can be divided into three episodes. An initial episode of relatively rapid cooling and unroofing (Late Permian‐Early Jurassic) is attributed to flexural rebound of the foreland in response to erosional removal of Alleghanian topographic load. Initial unroofing rates are higher in eastern Pennsylvania than in the west, consistent with a flexural model. An episode of little to no unroofing (Middle Jurassic‐late Oligocene) began contemporaneously with the inception of drift at the Atlantic continental margin. At this time, unloading of the orogen was replaced by subsidence and sedimentation on the new margin. Without flexural rebound the driving force for unroofing of the basin was removed and unroofing slowed greatly. An episode of rapid unroofing over the full width of the basin occurred from the Miocene to the present. Although the driving mechanism for unroofing at this time has not been identified, it is consistent with increased sedimentation rates in the middle Atlantic offshore basins for the same period.

Mineral weathering and elemental transport during hillslope evolution at the Susquehanna/Shale Hills Critical Zone Observatory

“…These combined observations suggest that the structural setting below SSHO is complex with gently to steeply dipping and faulted strata. Characteristic erosion rates in the Appalachian Mountain region range from 8 to 29 m/Myrs with northern shale units (e.g., the SSHO) near the lower limit of this range (Roden and Miller, 1989;Blackmer et al, 1994;Reuter et al, 2004).…”

Alpha thermochronology of carbonates

“…The region was then covered by a sequence of Paleozoic and Mesozoic strata (Volkert and Drake, 1999). Apatite fission-track ages from the central Appalachians range from 250 to 125 Ma (Roden and Miller, 1989;Blackmer et al, 1994). Therefore the apparent age of $127 Ma seems a quite reasonable result based on regional stratigraphy and thermochronology.…”

Quantifying Cretaceous–Cenozoic exhumation in the Otway Basin, southeastern Australia, using sonic transit time data: Implications for conventional and unconventional hydrocarbon prospectivity

“…In contrast to the two phases of exhumation observed in the Otway Basin, many unconventional plays in the United States appear to record only one significant exhumation event (Montgomery et al, 2005;Pollastro et al, 2011;Fall et al, 2012). The structural style of exhumation (i.e., passive margin inversion of synrift faults vs. foreland basin flexural rebound; Blackmer et al, 1994) is another key difference. Note that, according to Bowker (2007), Barnett Shale gas production is poorer in areas near faults and structural flexures (i.e., anticlines and synclines), such that their associated fractures, although rarely open, are detrimental to production.…”

“…The Upper Cretaceous Mesaverde Group in the southern Piceance Basin was exhumed by approximately 1800 m (∼5905 ft) during the Neogene (Nuccio and Roberts, 2003;Fall et al, 2012), whereas Pennsylvanian strata in the central Appalachian Basin have been exhumed by approximately 4400 m (∼14,435 ft) since the Late Permian (Blackmer et al, 1994;Reed et al, 2005). Although the timing of exhumation from maximum burial depths in the Fort Worth Basin is a matter of some debate (e.g., mid-Cretaceous; Montgomery et al, 2005;Late Triassic;Ewing, 2006), it is agreed that the Mississippian Barnett Shale has experienced more than 1500 m (∼4920 ft) of net exhumation (Jarvie et al, 2005Montgomery et al, 2005;Ewing, 2006).…”