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Geology topics

A.J. Froelich

Publications and source records attributed to A.J. Froelich.

At least 19 recordsLinked to original sources
USGS1995

Evolution of tholeiitic diabase sheet systems in the eastern United States: examples from the Culpeper Basin, Virginia-Maryland, and the Gettysburg Basin, Pennsylvania

High-TiO2, quartz-normative (HTQ) tholeiite sheets of Early Jurassic age have intruded mainly Late Triassic sedimentary rocks in several early Mesozoic basins in the eastern US. Field observations, petrographic study, geochemical analyses and stable isotope data from three HTQ sheet systems were used to develop a general model of magmatic differentiation and magmatic-hydrothermal interaction for HTQ sheets. The three sheet systems have remarkably similar major-oxide and trace-element compositions. Cumulus and evolved diabase in comagmatic sheets separated by tens of kilometers are related by igneous differentiation. Differentiated diabase in all three sheets have petrographic and geochemical signatures and fluid inclusions indicating hydrothermal alteration beginning near magmatic temperatures and continuing to relatively low temperatures. Sulfur and oxygen isotope data are consistent with a magmatic origin for the hydrothermal fluid. -from Authors

Pennsylvania, Maryland, Virginia↗
USGS1995

Geology of the Waterford quadrangle, Virginia and Maryland, and the Virginia part of the Point of Rocks quadrangle

The bedrock geology of the Waterford quadrangle and of the Virginia part of the Point of Rocks quadrangle consists of a portion of the Middle Proterozoic basement core and its cover sequence on the eastern limb of the Blue Ridge anticlinorium and the adjacent early Mesozoic Culpeper basin. The three major rock associations in this area are: 1) Middle Proterozoic gneisses intruded by late Proterozoic metadiabase dikes, 2) unconformably overlying late Proterozoic and early Paleozoic metavolcanic and metasedimentary rocks, and 3) Upper Triassic sedimentary strata intruded by early Jurassic diabase. The Triassic and Jurassic rocks are separated from the older rocks by a major normal fault, the Bull Run fault. Late Cenozoic surficial deposits of three major types unconformably overlie the bedrock: colluvium derived from Catoctin Mountain, terrace deposits of the Potomac River, and flood plain alluvium of the Potomac River and its tributaries. -from Authors

Maryland, Virginia↗
USGS1993

Emplacement and differentiation of the york haven diabase sheet, Pennsylvania

Many of the high-Ti quartz-normative tholeiitic intrusive sheets in the early Mesozoic rift basins of the Eastern USA exhibit lateral differentiation from mafic cumulate units, through diabase, to relatively evolved iron-rich rock types. We have investigated a representative example in detail, the York Haven sheet in the Gettysburg basin of south-central Pennsylvania. It ranges in thickness from 330 m to 675 m, and we have sampled it from base to top along four separate stratigraphic sections evenly spaced over the extent of the intrusion. The easternmost section (York Haven) is entirely basaltic bronzite cumulate (average 15 vol. % bronzite), whereas the westernmost (Reesers Summit) consists of diabase and low-MgO diabase with a middle to upper 'sandwich zone' of ferrogabbro. The intervening sections feature rock types transitional between the two end-member sequences. Chemically, the rock series shows a gradual east to west depletion of compatible elements (Mg, Ca, Ni, and Cr), and enrichment of incompatible elements [Ti, Fe, Na, K, P, Cu, Zr, Th, Ta, Hf, Sb, Cs, As, platinum group elements (PGEs), and rare earth elements (REEs)].We suggest two main processes for the trends observed in the York Haven sheet. First, flow differentiation during ascent and lateral injection of the parental magma produced a tongue of basaltic bronzite cumulate that thins from southeast to northwest and passes laterally into diabase, and, at the distal end of the intrusion, into low-MgO diabase. Then, in the latter stages of crystallization, densitydriven hydrothermal fluids transported incompatible elements westward, into structurally higher parts of the intrusion. Reaction of this residual aqueous fluid with partly crystallized low-MgO diabase produced a zone of ferrogabbro rich in hydrothermal replacement products (e.g., Cl-amphibole, biotite, ferrohypersthene, and skeletal ilmenite) and precipitates (e.g., quartz, fayalite, Cl-apatite, sulfides, and PGE minerals). © 1993 Oxford University Press.

Pennsylvania↗
USGS1990

Fractionation of palladium and platinum in a Mesozoic diabase sheet, Gettysburg basin, Pennsyvania: Implications for mineral exploration

The York Haven diabase sheet displays clear-cut evidence of fractionation of Pd and Pt during differentiation of a high-Ti (about 1.1%) quartz-normative tholeiitic magma (York Haven type). At York Haven the sheet is about 750 m thick. It is characterized by abundant cumulus MgO-rich orthopyroxene (bronzite), and is markedly depleted in incompatible elements relative to the chilled margins. In contrast, at Reesers Summit, 16 km to the northwest, the sheet is about 500 m thick and consists of evolved rocks that have contents of incompatible elements two to three times greater than in the enclosing chilled margins. These evolved rocks represent complementary fractions to the cumulate rocks at York Haven. Mineralogic, petrologic and geochemical variations suggest considerable lateral migration and fractionation of the initial magma. Chilled margins of both sections have essentially the same Pd and Pt contents (10 ppb each) and similar Pd to Pt ratios (1.2). During differentiation, the cumulate rocks at York Haven were enriched in Pt and depleted in Pd, whereas at Reesers Summit, the low-MgO diabase and ferrogabbro zone were enriched in Pd relative to Pt. Anomalously high contents of Pd (to 165 ppb), Au (to 54 ppb), and Te (to 26 ppb) were found in an iron- (to 18%) and chlorine- (to 0.44%) rich ferrogabbro at Reesers Summit, suggesting possible late or post-magmatic enrichment of precious metals. Field relations, geochemical and petrographic data provide guides for further exploration for Pd and Pt in differentiated high-Ti quartz-normative diabase sheets. Based on present information, the most favorable sites for economic deposits are late-stage differentiates enriched in Fe and Cl. ?? 1990.

Pennsylvania↗
USGS1989

Triassic-Jurassic stratigraphy of the Culpeper and Barboursville basins, Virginia and Maryland

The presence of Late Triassic and Early Jurassic spores and pollen throughout an exposed stratigraphic section has enabled the systematic palynofloral zonation of the entire Culpeper Group, supported in places by characteristic fish zones. Strata of the Culpeper Group range in age from the Late Triassic to the Early Jurassic. The distribution of strata in the Culpeper Group is explained by accumulation of clastic sediments in a closed basin flanked by fault-block mountains that were episodically uplifted and eroded. Deposition within the basin was controlled by subsidence accompanying extensional tectonics during a dominantly semiarid climate punctuated by periods of abundant precipitation. During the late stages of Jurassic deposition of the Culpeper Group, tholeiitic diabase stocks, sills, and dikes (about 195 Ma) extensively intruded the sedimentary rocks and basalt flows and thermally metamorphosed them. The strata in the Culpeper and Barboursville basins were regionally tilted to the west, particularly along the western border faults, which produced steep dips, drag folds, gravity slumping, and slippage along bedding planes; broad, gently southwest plunging folds, and en echelon and strike-slip faults along the western basin margin; and folds and transverse faults within the basins. -from Authors

Virginia, Maryland↗
USGS1988

Studies of the early Mesozoic basins of the eastern United States

The papers collected in this volume are extended versions selected from material given as oral and poster presentations at a workshop called "Geology of the Early Mesozoic Basins of Eastern North America,' held at the National Center of the US Geological Survey, May 11-14, 1987. The section titles are: 1) studies related to early Mesozoic sedimentary rocks, 2) studies related to Jurassic igneous rocks, 3) studies related to geophysics, structure, and tectonics, 4) studies related to mineral resources, and 5) fieldtrip guides to the Culpeper basin, Virginia. -from Editors

Bulletin↗