The Late Silurian-Early Devonian Helderberg Group, Central Appalachians, is a sequence of mixed siliciclastic-carbonate sediments that was deposited during relative tectonic quiescence on a ramp that built out from low-relief tectonic highlands bordering the eastern side of the Appalachian Basin. Three transgressive-regressive sequences are recognized. Each sequence was deposited over 2-3 m.y.; subsidence rates during deposition were 1 to 2 cm/1000 years. Skeletal grainstone/rudstone formed fringing skeletal banks that formed during regression and prograded away from the eastern side of the basin. Thick Middle Devonian siliciclastic sediments buried the Helderberg Group and updip subaerial exposures accompanying the onset of the Acadian Orogeny.
Cementation of the Helderberg Group began on the seafloor, but most cements formed under shallow (<300 m depth) to deep burial (300 m to 4 km) conditions. Regional cathodoluminescent zonation patterns in early, clear calcite cements indicate meteoric groundwaters, that become progressively more reducing away from recharge areas, were involved in shallow burial cementation. Progressive downdip reduction of meteoric groundwaters resulted in updip nonluminescent calcite cements that pass downdip into timecorrelative "subzoned" dull cement and finally, nonzoned dull cements. Calculated stable isotopic compositions of Helderberg shallow burial pore fluids are similar to values in modern coastal meteoric groundwaters. Extensive meteoric groundwater systems developed over a 3-4 m. y. period when the Helderberg Group was subaerially exposed along the eastern basin margin and when Helderberg aquifers were confined by fine-grained sediments at <300 m burial depth. Meteoric groundwaters had recharge areas in eastern tectonic highlands which supplied sufficient hydraulic heads to expel connate marine pore fluids and discharge at least 150 km offshore onto the floor of the Appalachian Basin.
Void-filling dull calcite cement formed from deep burial (300 m to 4 km) pore fluids with calculated chemical compositions similar to modern oil field brines. Migration of hydrocarbons and high-temperature, high-pressure brines occurred during Late Paleozoic deformation after Helderberg sediments were totally cemented. Brines probably came from eastern overthrusted terranes and migrated through fractures without altering conodont CAI values. Late hydrocarbons probably had several source rocks. / Ph. D.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/76072 |
| Date | January 1984 |
| Creators | Dorobek, Steven L. |
| Contributors | Geological Sciences, Read, J. Frederick, Bambach, Richard K., Craig, James R., Eriksson, Kenneth, Rimstidt, J. Donald |
| Publisher | Virginia Polytechnic Institute and State University |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | en_US |
| Detected Language | English |
| Type | Dissertation, Text |
| Format | x, 237 leaves, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
| Relation | OCLC# 11888346 |
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