Abstract The Hartford-Deerfield basin, a Late Triassic to Early Jurassic rift basin located in central Connecticut and Massachusetts, is the northernmost basin of the onshore Mesozoic rift basins in the eastern United States. The presence of asphaltic petroleum in outcrops indicates that at least one active petroleum system has existed within the basin. However, to-date oil and gas wells have not been drilled in the basin to test any type of petroleum trap. There are good to excellent quality source rocks (up to 3.8% present day total organic carbon) within the Jurassic East Berlin and Portland formations. While these source rock intervals are fairly extensive and at peak oil to peak gas stages of maturity, individual source rock beds are relatively thin (typically less than 1 m) based solely on outcrop observations. Potential reservoir rocks within the Hartford-Deerfield basin are arkosic conglomerates, pebbly sandstones, and finer grained sandstones, shales, siltstones, and fractured igneous rocks of the Triassic New Haven and Jurassic East Berlin and Portland formations (and possibly other units). Sandstone porosity data from 75 samples range from less than 1% to 21%, with a mean of 5%. Permeability is equally low, except around joints, fractures, and faults. Seals are likely to be unfractured intra-formational shales and tight igneous bodies. Maturation, generation, and expulsion likely occurred during the late synrift period (Early Jurassic) accentuated by an increase in local geothermal gradient, igneous intrusions, and hydrothermal fluid circulation. Migration pathways were likely along syn- and postrift faults and fracture zones. Petroleum resources, if present, are probably unconventional (continuous) accumulations as conventionally accumulated petroleum is likely not present in significant volumes.

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.

Flood basalts erupted at three distinct times during formation of the Hartford basin. Such tripartite activity is believed to be inherent to most continental rift-associated magmatism. In the Hartford basin, lavas erupted from three northeast-trending dikes. First, the Talcott basalt erupted from the eastern Higganum dike; 138 ka later the Holyoke basalt erupted from the central Buttress dike; and 345 ka later still, the Hampden basalt erupted from the western Bridgeport dike. The time between eruptions is estimated from Milankovitch-type cycles in lake sediments between the flows. It is proposed that magmas were generated by decompression melting of the adiabatically rising mantle beneath the basin. All three basalts rose through a common conduit in the lower lithosphere, but the upper part of this dike was repeatedly beheaded and shifted eastward by crustal extension that occurred along an eastward-dipping detachment surface that passed beneath the basin. Extension rates determined from dike displacements range from 4 to 9 cm/yr, being greatest where the basin is widest. The compositional changes and decreases in eruption temperature of successive basalts are thought to result from adiabatic partial melting of the source region as it continued to rise beneath the basin. The 18 °C decrease in eruption temperature (experimentally determined) between the Talcott and Holyoke basalts can be attributed to the Holyoke magma being formed by ~4% partial melting of the mantle. Similarly, the Hampden basalt would be formed by 2.2% partial melting to give a 9 °C decrease. If 1% melt must remain in the mantle, the fractions of partial melt that formed these two basalts are in the same proportions as the estimated erupted volumes of these basalts. Each of the basalts has a calculated magma density that is significantly less than average crustal densities. Consequently, magma would not have ponded at depth but would have risen rapidly toward the surface. Because the magmas were dry, the partial melting that began in the mantle continued all the way to the surface. During ascent, therefore, the decreasing crystal content of the magma caused the viscosity to decrease exponentially and the bulk density to decrease linearly. Moreover, the volume expansion on melting added to the buoyant force causing the magma to rise. These factors combined to give high magmatic fluxes, which account for the enormous volumes of single eruptive units. Because of the rapid ascent of the magmas, little intratelluric differentiation could occur. The major compositional differences between successive basalts must therefore reflect differences in the proportions of melt to solid in the magmas rising from the source or to phase changes resulting from decompression of the ascending source region. Resorption of phenocrysts and filter pressing during ascent modified magma compositions sufficiently to produce rocks that plot near multiple saturation boundaries. The magmas were also contaminated by low-melting fractions of crustal rocks during turbulent emplacement through wide dikes.

This chapter reviews paleomagnetic data on igneous rocks in the Culpeper, Newark, and Hartford/Deerfield basins, beginning with the work of DuBois and others (1957) and extending through 1990. Data from published articles is supplemented with unpublished data from theses and individual projects currently in progress. Analysis of these data support Prevot and McWilliams’ (1989) correlation of the Orange Mountain, Preakness, and Hook Mountain Basalts with the Talcott, Holyoke, and Hampden Basalts, respectively. Their correlation of the Deerfield Basalt with the Holyoke Basalt and the Preakness Basalt is also supported, and the unpublished data presented here support extending this correlation to the Sander Basalt in the Culpeper basin. Additionally, it is suggested that the anomalous magnetization of this group of rocks may provide evidence of an excursion of the magnetic pole. The data presented in this review do not support the separation of igneous activity into two separate events as suggested by Smith and Noltimier (1979). A single period of igneous activity producing both intrusive and volcanic rocks and restricted to a relatively short period of time at the beginning of the Jurassic seems more likely. Furthermore, the N1 and N2 poles of Smith and Noltimier (1979) are not representative of Newark Supergroup igneous rocks and that they should be abandoned in calculating North American apparent polar wander paths.