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| Duffer Formation | |
|---|---|
| Name | Duffer Formation |
| Type | Geological formation |
| Period | Late Ordovician |
| Lithology | Sandstone, shale, siltstone, limestone |
| Namedfor | Duffer Creek |
| Namedby | A. B. Cartwright, 1958 |
| Region | Northwestern Appalachia |
| Country | United States, Canada |
| Underlies | Beech Fork Formation |
| Overlies | Maple Run Formation |
| Thickness | 50–320 m |
Duffer Formation is a stratigraphic unit composed primarily of siliciclastic and carbonate rocks exposed in parts of the northwestern Appalachian Basin and adjacent foreland provinces. The formation records Late Ordovician deposition during a time of active tectonism and eustatic change and preserves a diverse fossil assemblage that has been studied in paleontology, stratigraphy, and basin analysis. Work on the unit has involved researchers from institutions across North America and has contributed to correlations with successions in the Cincinnati Arch, Michigan Basin, and Maritime Provinces.
The Duffer Formation consists of interbedded sandstone, siltstone, shale, and nodular limestone with subordinate conglomerate horizons. Typical sections display fining-upward sequences comprising cross-bedded quartz-rich arenite passing into mudstone and thin-bedded bioclastic packstone and wackestone. Diagenetic overprints include calcite cementation and illitization of volcanic ash layers correlated to ash beds in the Bromide Formation and Trenton Group. Sedimentary structures described by field parties from the United States Geological Survey, Ohio Geological Survey, and Geological Survey of Canada include planar and trough cross-stratification, ripple lamination, soft-sediment deformation, and load casts comparable to those in the Queens Formation and Cobleskill Limestone.
Biostratigraphic control ties the Duffer Formation to the Late Ordovician, particularly the Katian to Hirnantian stages, via conodont zonation and graptolite correlations with faunas documented in the Clifton Formation and Grafton Member. Chemostratigraphic excursions in carbon isotopes have been correlated with the Hirnantian glaciation event recorded in the Karakaya Basin and sections studied in the Antarctic Peninsula. Regionally, the Duffer Formation conformably overlies the Maple Run Formation and is conformably to disconformably overlain by the Beech Fork Formation, enabling links to sequence stratigraphic models developed for the Appalachian Basin and Michigan Basin.
The formation yields a diverse assemblage of marine invertebrates including brachiopods (e.g., taxa comparable to those described from the Cincinnatian Series), trilobites with affinities to genera recorded in the St. George Group, bivalves, gastropods, bryozoans, and echinoderm fragments. Conodont assemblages, including elements similar to those reported from the Trenton Limestone, provide primary age constraints, while graptolites comparable to species from the Hartfell Shales inform paleoecologic reconstructions. Trace fossils such as Cruziana-like burrows and Skolithos vertical tubes occur in high-energy sandstone units and have been compared to ichnofacies documented in the Burgess Shale-age sequences and Ordovician ichnological studies from the Midland Valley. Occurrences of microbial laminites and small shelly fossil fragments have prompted comparisons with early Paleozoic localities in the Tremadocian successions, though the Duffer material is distinctly Katian–Hirnantian in age.
Sedimentologic and paleontologic evidence indicates deposition across a distal to proximal shallow-marine ramp with episodes of storm-dominated clastic input, tidal reworking, and periodic deeper-water condensation. Facies analysis aligns high-energy sandstone lobes with tempestite models developed for the Cincinnati Arch and storm-sheet deposits recorded in the Baltic Basin, whereas finer siliciclastic and carbonate facies reflect quieter outer ramp and slope settings comparable to environments described from the Anticosti Island successions. Tectonic subsidence related to the Taconic Orogeny and eustatic sea-level fluctuations tied to Late Ordovician glaciation influenced accommodation and sediment supply, consistent with models applied to the Climatic Revolution interval.
The Duffer Formation is mapped across parts of northeastern Ohio, northwestern Pennsylvania, southwestern New York (state), and extends into southeastern Ontario and adjacent portions of Quebec (province) in outcrops and subsurface wells. Classic exposures occur in roadcuts, stream banks, and quarry walls along the Allegheny Front and within anticlines studied by teams from Ohio State University, University of Toronto, and the Pennsylvania Geological Survey. Subsurface correlations rely on boreholes logged by the United States Bureau of Mines and provincial petroleum surveys, permitting ties to equivalent Ordovician packages in the Michigan Basin and offshore wells in the Gulf of St. Lawrence region.
The name was introduced in 1958 by A. B. Cartwright in mapping reports produced for the United States Geological Survey following regional field campaigns prompted by mineral-resource assessments. Subsequent stratigraphic refinement and paleontological inventories were carried out by researchers affiliated with the American Museum of Natural History, Royal Ontario Museum, and university geology departments, leading to detailed lithostratigraphic descriptions in the 1970s and revised biostratigraphic correlations during the 1990s. International collaborations connected Duffer studies with global Ordovician research programs such as the International Commission on Ordovician Stratigraphy and symposiums hosted by the Geological Society of America and Geological Association of Canada.
Category:Ordovician geology Category:Geologic formations of the United States Category:Geologic formations of Canada