Geology topics

R. T. Faill

Publications and source records attributed to R. T. Faill.

5 recordsLinked to original sources

USGS2005

Reply

In summary, the rift model as presented by Schlische and Withjack (2005) is related only to the Doylestown subbasin - they do not discuss the lithosome relationships throughout the remainder of the basin. But, even in the Doylestown subbasin, their model raises more questions than it answers. They have not demonstrated that syndepositional faulting actually occurred - instead, they only present evidence to support a preconceived fault model, and even some of the evidence they cite is inconsistent with their rift hypothesis. Rift tectonism is not a good m odel for the Birdsboro basin, as seductively simple as such a model may seem. Many aspects of the basin are not explained by rifting. That is why the focus in the original Birdsboro basin paper (Faill, 2003) was only on the sediments, their distributions, and the inferences that could be made therefrom. Discussion of how the basin formed and the overall causative tectonics was eschewed in order to avoid the introduction of preconceived notions (half-graben, rift tectonism). It is hoped that a better understanding of the sediment content and its distribution will help in developing a tectonic model that fully explains the origin, development, and subsequent history of the entire Birdsboro basin.

Geological Society of America Bulletin↗

USGS2003

The early Mesozoic Birdsboro central Atlantic margin basin in the Mid-Atlantic region, eastern United States

The early Mesozoic Birdsboro basin (new name) was a single, elongate depositional trough in the present Mid-Atlantic area of the eastern United States, extending north-eastward from central Virginia across Maryland, Pennsylvania, and New Jersey into southern New York. What now remains after erosion comprises the Barboursville, Culpeper, Gettysburg, and Newark remnants. Some 7?? km of late Triassic and early Jurassic continental sediments of varying provenances entered and spread across in the Birdsboro basin in several depositional environments. The five resulting sedimentary lithosomes include feldspathic sandstone, quartzose sandstone, red silty mudstone, gray shale, and fanglomerate. The extensive interbedding, intertonguing, and lateral gradation among these lithosomes suggest that they were contemporary and closely interrelated. The feldspathic sandstone lithosome contains sediment with a southeastern provenance that accumulated in a bajada environment extending the length of the southeastern side of the basin. Sediment in the quartzose sandstone lithosome had a northwestern provenance-the coarse-grained fraction formed regional alluvial fans at the mouths of four major input centers. The finer-grained fraction was deposited in the distal reaches of these fans and in the playa environments in the interfan areas; this fraction formed the red silty mudstone lithosome. Gray/black shales and argillites of the gray shale lithosome accumulated in lacustrine environments in the interfan areas. The fanglomerate lithosome comprises numerous small, lobate deposits of poorly sorted sediment along both basin margins. The location and time of activity of the northwest input centers largely determined the distribution and areal extent of the various depositional environments and consequent lithosome along the length and across the width of the basin. The Birdsboro basin was deformed (tilted, faulted, and folded) sometime after the deposition of the youngest preserved rocks (early Sinemurian). The deformation varied along the length of the basin, producing differences in the amount of tilting, structural elevation, and subsequent erosion. The present erosional remnants create the illusion of four originally separate depositional basins.

Geological Society of America Bulletin↗

USGS1998

A geologic history of the north-central Appalachians, part 3. The Alleghany orogeny

The north-central Appalachians occupy a critical position within the 3000+ km-long Appalachian orogen, lying southwest of the boundary between the central and northern Appalachians (CNAB). The one-billion-year-long history of tectonic activity in eastern Laurentia includes the creation and evolution of the Appalachian orogen during the Paleozoic and the Mesozoic transformation of the orogen into a passive margin during Pangea's disassembly. A most important ingredient in the evolution of the orogen was the Alleghany orogeny, which was driven by the convergence and collision between Laurentia (Laurussia) and West Gondwana (Africa). The Alleghany orogeny in the central and southern Appalachians was a decollement tectonism that involved a larger part of eastern Laurentia than had the previous three orogenies. The fundamental element was a very low-angle thrust (decollement) that originated in mid-crustal levels east of the presently-exposed Appalachians and rose westwardly to progressively higher levels in the upper crust and the supra-crustal Paleozoic section. Alleghany deformation was widely developed in the hanging-wall block (allochthon), primarily in the form of thrust faults and fold-and-thrust structures, both of which splayed upward from the basal decollement. The youngest manifestations of the Alleghany orogeny were northeast-trending strike-slip faults and dextral shear zones in the Piedmont. In the north-central Appalachians, the exposed allochthon consists of two parts: the sedimentary externides (Appalachian Plateau and Valley and Ridge provinces) and the crystalline externides (Reading Prong, Blue Ridge belt, and Piedmont province). Long, thrust-cored anticlines predominate in the sedimentary externides. A widespread layer-parallel shortening preceded the folding; it is largely coaxial with the folding but extends considerably farther to the northwest toward the craton. It is hypothesized that the folding developed in reverse order, sequentially from the northwest to the southeast. The crystalline externides are dominated by low-angle thrust faults and upright folds trending east-northeast. The first-order Valley and Ridge folds on the northwest side acted as a buttress and diverted the crystalline externides rocks north-northwestwardly, onto the topographic low area over the Anthracite region. This thrusting of the crystalline externides caused anthracitization of the coals within the Pennsylvanian rocks there. Metamorphism and magmatism were significant events during the earlier phase of the Alleghany orogeny in the southern Appalachians. Whatever magmatism and medium-to high-grade metamorphism developed in the north-central Appalachians are in the covered internides to the southeast. The Alleghany orogeny of the north-central Appalachians occurred during the Early Permian. Erosion of anticlinal crests probably began as the folds grew, with accumulation of this locally-derived sediment in the intervening synclines. A regional alluvial plain coalesced above the partially-eroded externides structures as erosion of the pre-Alleghany highland and the Alleghany hinterland mountains continued to the southeast, spreading sediment to the northwest. This erosion and northwest transport probably persisted, with diminishing intensity, throughout the remainder of the Permian and into the Mesozoic, and changed only with the beginning of crustal extension during the Late Triassic.

American Journal of Science↗

USGS1997

A geologic history of the north-central Appalachians. Part 1. Orogenesis from the Mesoproterozoic through the Taconic orogeny

The north-central Appalachians lie just southwest of the boundary between the central/southern and northern Appalachians, occupying a critical position within the 3000+ km-long Appalachian orogen. The history of tectonic activity in the north-central Appalachians spans more than one billion years, from the assembly and breakup of a Neoproterozoic supercontinent, through active orogenesis during Laurentia's Paleozoic northward journey off the western margin of West Gondwana, to the Mesozoic transformation of the active orogen into a passive margin during Pangea's disassembly. The major tectonic events include five compressional orogenies and two extensional episodes: 1. The late Mesoproterozoic Grenville orogeny assembled several continental masses into the Neoproterozoic supercontinent, Rodinia. It was the most widespread but now least exposed tectonism, and rocks involved in this event underlie as basement most of the exposed north-central Appalachians. Large fragments of Laurentian Grenville rock were subsequently broken off and incorporated in later Appalachian orogenesis. 2. Crustal extension and rifting late in the Neoproterozoic and into the earliest Cambrian separated Laurentia from West Gondwana, thereby forming the intervening Theic ocean and two continental rifts on Laurentia's eastern margin, the Catoctin rift and, later in the Middle? to Late Cambrian, the so-called "Rome" trough. The initial siliciclastic sedimentation on the margin migrated westward through time onto the craton, supplanted by a thick and increasingly wide carbonate shelf on Laurentia's eastern margin. Two microcontinents of Grenville-age, non-Laurentian(?) continental rock became positioned east of Laurentia, thereby creating the Octoraro sea as an arm of Theia. 3. In Theia, east of the two microcontinents, magmatic arcs developed over a subduction zone late in the Cambrian. Convergence within Theia caused the Potomac orogeny, which obduced the arcs (Wilmington Complex, Cecil Amalgamate) over the microcontinents and associated Theic deposits (including accretionary wedge sediments). 4. Continued westward convergence collapsed the Octoraro sea, producing the Middle to Late Ordovician Taconic orogeny in which the Potomac-deformed magmatic arcs and associated Theic elements were obduced onto the Laurentian continental margin. This obduction: drowned the carbonate shelf with siliciclastic sediments (Martinsburg Formation); drove continental rise and basinal deposits over the carbonate shelf on the Martic thrust; slid the Hamburg klippe onto the shelf; and accreted the Potomac package of microcontinent/arc/basinal-sediments onto the Laurentian margin. This orogeny transformed the broad Early Paleozoic carbonate shelf into the Appalachian basin that persisted throughout the Middle and Late Paleozoic. 5. The Middle Devonian Acadian orogeny ended the largely paralic environment that dominated the Appalachian basin during the Late Silurian. Active orogenesis in New England probably extended southward to the north-central Appalachians, because a vast amount of terrigenous sediment was introduced into the Appalachian basin to form the Catskill delta; however, evidence of actual Acadian deformation and metamorphism is lacking at this latitude. These presumed internal Acadides have yet to be found. 6. The convergence of West Gondwana and Laurentia during the Late Carboniferous and earliest Permian produced the Permian Alleghany orogeny in the north-central Appalachians. This widespread decollement tectonism directly affected a larger area of the presently exposed central and southern Appalachians than any earlier Paleozoic tectonic event. An early layer-parallel shortening phase gave way to a fold-thrust development above a basal decollement. This Alleghany fold-and-thrust tectonism created long, arcuate folds in the Appalachian basin. Late in the Alleghany orogeny, rock thrust northward over the Carboniferous rocks in the Anthracite region of northeastern Pennsylvania caused anthracitization of the underlying coals. The internal Alleghanides are not presently exposed. 7. Crustal extension in the Late Triassic and Early Jurassic produced numerous local, closed basins along eastern North America. Igneous intrusions and effusions marked the beginning of the Jurassic. By the end of the Early Jurassic, horizontal crustal rebound in response to opening of the Atlantic Ocean rotated the basins by crustal inversion, which folded some within-basin rocks and produced a prominent topographic ridge along the Piedmont, up-dip of the basins. Subsequent erosion of this Piedmont ridge and other parts of the Appalachian orogen fed large volumes of sediment to offshore basins during the remainder of the Mesozoic and throughout the Cenozoic. Each of these orogenies affected most of the Appalachian orogen. The tectonic expression of each orogeny varied along and across the orogen. The elements and structural bodies involved in each also varied along strike. However, many common elements persist from one part of the orogen to another; only a few features are singular in time and space. The tectonic boundary between the central and northern Appalachians is one of these singular features-it is solely an Alleghanian artifact.

American Journal of Science↗

USGS1997

A geologic history of the north-central Appalachians, part 2: The Appalachian basin from the Silurian through the Carboniferous

The north-central Appalachians occupy a critical position within the 3000+ km-long Appalachian orogen, lying southwest of the boundary between the central and northern Appalachians (CNAB). The one-billion-year-long history of tectonic activity in the north-central Appalachians includes the assembly and breakup of a late Proterozoic supercontinent, the creation and evolution of the Appalachian orogen during the Paleozoic, and the Mesozoic transformation of the active orogen into a passive margin during Pangea's disassembly. An important part of the Middle and Late Paleozoic history is the formation and growth of the Appalachian basin, an enormous, elongate continental basin lying cratonward of the active Appalachian internides along Laurentia's eastern margin. The Appalachian basin developed out of the Taconic orogeny, the second of the four orogenies that formed and modified the character of the Appalachian orogen. Prior to the Taconic orogeny, the eastern margin of Laurentia consisted of a broad carbonate shelf facing the Theic Ocean. The Taconic orogeny obduced various Theic components, including microcontinents, magmatic arcs, and accretionary prisms, onto the carbonate shelf. The resulting Taconic highlands formed a topographic barrier between Theia and the craton's interior. Because tectonic activity in the internides continued more or less throughout the remainder of the Paleozoic, the Appalachian basin never had a direct connection with Theia. The coarse-grained molasse from the newly uplifted Taconic highland spread westward over most of the basin during the Early Silurian. Carbonate deposition gradually encroached eastward over the basin as the siliciclastic input from the southeast waned. A resurgence in the earliest Late Silurian, perhaps related to orogenesis in Newfoundland and the Maritimes, expanded the clastic wedge somewhat. Before long, however, carbonate deposition once again dominated most of the north-central basin for the remainder of the Silurian and into the Early Devonian. The Early-to-Middle Devonian Acadian orogeny began introducing siliciclastic material into the eastern part of the Appalachian basin, thereby ending the largely paralic environment that persisted from the Late Silurian. A number of deltas formed around local sediment-input centers during the Middle Devonian. Continued uplift and/or orogenesis must have continued into the Late Devonian because a vast amount of terrigenous sediment was introduced into the Appalachian basin to form the vast Upper Devonian Catskill delta. Although the principal tectonic activity appears to have centered in New England, the large volume of sediment input suggests that orogenesis may have extended southward to the north-central Appalachians. However, direct evidence of actual Acadian deformation and metamorphism is lacking at this latitude. The north-central Appalachian basin underwent a significant change during the Carboniferous. Prior to this time, the shoreline shifted laterally through transgressions and regressions largely as a function of sediment input and regional subsidence. In the Carboniferous, vertical movements in the basin became more important. During the Early Carboniferous, much of the Mauch Chunk delta was eroded in the central and western parts of the basin. At the end of the Early Carboniferous, an unconformity truncated progressively older rocks to the north toward New York. New sediment transport patterns and depositional environments within the basin during the Late Carboniferous probably reflect significant topographic and tectonic changes, not only in the hinterland, but in the basin itself, perhaps in conjunction with climatic changes. Extensive interfluvial swamps on a lower delta plain accumulated thick organic deposits which would become coal. This delta plain graded southwestward into tidal and marine environments. The Alleghany orogeny in the Early Permian interrupted deposition in much of the north-central Appalachian basin and profoundly altered its structure.

American Journal of Science↗