Abstract The Richmond basin, a rift basin of Late Triassic to Early Jurassic age in east-central Virginia, produced the first coal mined in the United States in the early 1700s. These Triassic coal beds are thick and gas-rich, and fatal explosions were common during the early history of exploitation. Since 1897, at least 38 confirmed oil, natural gas, and coal tests have been drilled within the basin. Although shows of asphaltic petroleum and natural gas indicate that active petroleum systems existed therein, no economic hydrocarbon accumulations have been discovered to-date. The Richmond basin has been assessed by the U. S. Geological Survey (USGS) as one composite total petroleum system, in which the hydrocarbon potential of the source beds (both coal and dark shale) and potential reservoirs have been combined into a single continuous tight gas assessment unit within the Chesterfield and Tuckahoe groups (Upper Triassic). Sandstone porosities are generally low (<1 % to 14 %). Thick, dark-colored shales have total organic carbon (TOC) values that range from <1% to 10%, and vitrinite reflectance (%R O ) values that range generally from about 0.3 to 1.1%, which indicates that the submature to super mature shales appear to be the source of the hydrocarbons recovered from some of the boreholes. The stratigraphic combination of these potential source rocks, tight sandstones, and hydrocarbon shows are the basis for the current USGS assessment of the technically recoverable undiscovered hydrocarbon resources of the basin. Mean values for these resources are 211 billion cubic feet of gas (BCFG) and 11 million barrels of natural gas liquids (MMBNGL).

Nonpedogenic calcrete is difficult to distinguish from pedogenic calcrete in the fossil record; both alpha and beta textures have been observed from fossil and modern examples. However, a calcrete from the New Haven Arkose (Hartford Basin, Connecticut) is shown here to be of a nonpedogenic origin through sedimentologic and petrographic evidence. An accumulation of thin sheets of displacive calcite layers found in a decimeter-thick horizon of anastomosing veins within the upper portion of a red mudstone is correlated to calcite cement found in the overlying sandstone. Based on petrography, we recognize six generations of calcite in the mudstone-sandstone hosts. The first five generations are associated with rhizoliths that can be related to deep taproots and are interpreted to have formed by precipitation from shallow groundwater. There are no vadose-type cement morphologies; the calcite has luminescent zones, indicating that Mn was soluble and thus oxygen levels were low. These cements clearly formed several meters below what would have been the surface of the channel sand body. We suggest that calcite cement stratigraphy combined with redox models for the behavior of Mn (as well as Fe and U) may aid in the identification of nonpedogenic versus pedogenic carbonates in the geologic record. Additionally, the calcite from this carbonate layer has been dated using the U-Pb method. Our results provide insight into the environmental and diagenetic fluid conditions favorable for providing a spread in U/Pb ratios that are suitable for precise dating of calcites in otherwise undateable sections. Las calcretas no-pedogénicas son difíciles de diferenciar de las pedogénicas en el registro antiguo; los dos tipos de textura alfa y beta se reconocen en depósitos antiguos y recientes. Sin embargo, en el caso concreto de la calcreta de New Haven Arkose (Cuenca Hartford, Connecticut), los datos sedimentológicos y las evidencias petrográficas permiten demostrar su origen no-pedogénico. La presencia, dentro de lutitas rojas, de finas capas de calcita desplazativa dentro de un horizonte decimétrico de venas anastomosadas se puede correlacionar con la formación de cemento calcítico en las areniscas infrayacentes a dichas lutitas rojas. Petrográficametne se reconocen seis generaciones de calcita en las lutitas y areniscas que constituyen el sustrato. Las cinco primeras generaciones están asociadas con rizolitos de sistemas radiculares profundos y se pudieron formar por precipitación a partir de aguas freáticas someras. No hay cementos vadosos y la calcita tiene zonas luminiscentes indicando que el Mn era soluble y, por tanto, los niveles de oxígeno bajos. Estos cementos se formaron claramente varios metros por debajo de lo que fue la superficie del canal de arenas. Sugerimos que la estratigrafía de los cementos de calcita combinada con los modelos redox de comportamiento de Mn (también Fe y U) pueden ayudar en la identificación de los carbonatos pedogénicos y no-pedogénicos en el registro geológico. Además la calcita de esta capa carbonática se ha datado por el método de U-Pb. Nuestros resultados permiten una mejor caracterización de las condiciones ambientales y de los tipos de fluidos más favorables para ampliar el rango de valores U/Pb necesarios para datar calcitas en secciones que de otro modo no se podrían datar.

Orbitally controlled, sedimentary cycles of the Newark Supergroup permit palyniferous Late Triassic sections to be calibrated in time. Carnian palynofloras from the Richmond basin exhibit 2-m.y. fluctuations in the spore/pollen ratio, but taxonomic composition remains stable. Diversity of Norian and Rhaetian palynofloras increases prior to a 60% reduction at the Triassic/Jurassic boundary. The extinction of Late Triassic palynomorph species is coincident with a spike in the spore/pollen ratio and approximately synchronous with the last appearances of tetrapod taxa and ichnofossil genera. This geologically brief episode of biotic turnover is consistent with bolide impact hypotheses.

The Upper Nyack (UN) section of the Palisades sill is comprised principally of pigeonite-augite and augite diabases with an interstitial (remnant) glassy mesostasis. Major- and trace-metal analyses of 110 diabases, 20 being analyzed for rare-earth elements (REE), provide data regarding the source of chemical variations within the UN section and suggest a unifying theory, the propsed C-T-D Model, for differentiation within the Palisades sheet. Preliminary work establishes that the chemostratigraphic horizons present in the type section at Englewood Cliffs (ENG)are broadly continuous from south to north and that important petrographic facies have undergone substantial mineralogic reorganization. The three most significant S → N transitions include: (1) holocrystalline subophitic chilled diabase (ENG) → hyalosiderite chilled diabase (Piermont) → glomeroporphyritic to intersertal chilled diabase (UN); (2) olivine zone (ENG) → (augite diabase) [UN]) → bronzite diabase (Nyack Beach State Park); and (3) fayalite granophyre and related “sandwich” horizon facies (ENG) → altered (porphyritic) augite ferrodiabase (UN). Bulk compositional and trace-metal plots establish that south to north petrologic tiers comprise a single differentiation series. Differentiation series data are consistent with three-stage model that involves early olivine fractionation followed by pyroxene- and plagioclase-dominated sequences. The cumulus horizons are modeled principally in terms of the accumulation of pyroxene-dominated fractionation products by varying proportions of olivine fractionated or later liquid. It is proposed that crystals collect in convection cells or like environments at deep levels within the sheet are partially remelted, resorted, and/or recrystallized during epizonal emplacement due to: magma decompression, shallow level flow differentiation, and magma–wall rock interactions. The Cumulus-Transport-Decompression (C-T-D) Model, based on these observations, is advanced to integrate multiple differentiation factors within a single interpretive framework for Palisades sill evolution.

Detailed petrographic and geochemical analyses of samples collected from eight cores of the U.S. Army Corps of Engineers Passaic Tunnel project indicate that the three principal basalt formations of the early Mesozoic Newark basin in New Jersey are composed of multiple flow units that can be identified on the basis of physical and petrochemical criteria. The high-titanium, quartz-normative (HTQ) Orange Mountain Basalt (lowermost formation: at least three flow units) and high-iron and high-titanium, quartz-normative (HFTQ) Hook Mountain Basalt (uppermost formation: at least two flow units) include flows characterized by a limited range in composition and are indicative of eruptive cycles during which individual magma sources underwent little change in composition and were tapped repeatedly. In contrast, the wide chemical diversity of the flows in the Preakness Basalt (middle formation: possibly nine flow units) is indicative of a period of volcanism in which successive eruptions were derived either from (1) separate high-iron, quartz-normative (HFQ) and low-titanium, quartz-normative (LTQ) parental magmas; or (2) an LTQ parent that changed composition during the eruptive interval. The HTQ composition and limited variability of the Orange Mountain Basalt indicate petrochemical correlation with the stratigraphically comparable Mount Zion Church (Culpeper basin, Virginia) and Talcott (Hartford basin, Connecticut) Basalts. The wide range in composition of the Preakness Basalt supports previous petrochemical correlation with the Hickory Grove and Sander Basalts of the Culpeper basin. The HFQ compositional type of the Preakness, Hickory Grove, and Sander Basalts is similar to the Holyoke Basalt of the Hartford basin (Connecticut and Massachusetts) and Deerfield Basalt of the Deerfield basin (Massachusetts). The HFTQ Hook Mountain Basalt is nearly identical to the stratigraphically comparable Hampden Basalt of the Hartford basin (Connecticut and Massachusetts) in both composition and limited degree of compositional variability. These new data and conclusions drawn from recent cyclostratigraphic investigations of the intercalated sedimentary strata from these basins define three nearly synchronous eruptive intervals for the Eastern North America (ENA) province from Virginia to Massachusetts. Volcanic interval I, which followed an extended period of sedimentation in the Culpeper, Newark, and Hartford basins, was marked by repeated production of exclusively HTQ-type basalt and involved parental magmas that did not significantly change composition during the eruptive episode. Volcanic interval II, which followed a second, shorter period of basin sedimentation occurring throughout the province, began with eruption of the Hickory Grove Basalt in the Culpeper basin and, following a period of renewed terrestrial sedimentation in this basin, resumed with nearly simultaneous extrusion of HFQ-type basalt in all of the basins. In the northern basins (Hartford and Deerfield), where basalt eruption was uninterrupted by significant sedimentation, this interval involved only HFQ-type basalt. However, in the southern basins (Culpeper and Newark), where basalt production was interspersed with one or more periods of sedimentation, lavas defined a range of compositions, including LTQ-type and other hybrid varieties. Volcanic interval III, which followed another significant period of sedimentation in the Newark and Hartford basins, involved production of exclusively HFTQ-type basalt from parental sources that underwent slight change in composition during the episode. The similarity of inferred parental magma compositions from Virginia to Massachusetts implies the effect of large-scale petrologic control on melt production. The time-composition relationships of the basalts indicate that the volcanic sequences of the ENA province were produced by episodic tapping of compositionally distinct parental sources that varied in the extent of volcanic production on both a regional and intrabasin scale.

Newark Supergroup rocks of eastern North America provide a fine-scale record of continental faunal and floral change spanning all of the Late Triassic and most of the Early Jurassic (Carnian/Pliensbachian). Lake-level cycles controlled by orbitally induced climate changes permit calibration of biotic change at a 20,000-year level of resolution. The patterns of both the faunal and floral changes suggest that the Triassic/Jurassic biotic changes consisted of a large number of extinctions without replacement: the succeeding assemblages consist of survivors. The Triassic/Jurassic Newark faunal and floral transition appears synchronous with the massive marine extinctions noted elsewhere in the world. Preliminary data suggest the Newark pollen and spore transition took place in less than 20,000 years (within one lake-level cycle), whereas available bone and footprint data suggest the extinction of ecologically dominant Late Triassic terrestrial vertebrates could have taken as long as 700,000 years. Although we are still in the early stages of investigation, what we know about the Newark Triassic/Jurassic episode is consistent with a very large and abrupt mass extinction event. The penecon-temporaneous Manicouagan impact still seems a plausible cause. A search has only now begun for evidence of an impact-derived fallout layer in the Newark Supergroup, but it is far too early to confirm or deny a causal role for a bolide impact for the Triassic/Jurassic extinctions, in the Newark Supergroup or elsewhere.