Abstract Paleosols in rift-related Upper Triassic (Camian-Norian) terrestrial deposits of the Durham sub-basin of North Carolina record evidence of a shift from wetter to drier conditions, and slightly elevated atmospheric levels of carbon dioxide. Paleosols in siltstone interbeds of Lithofacies Association IIa (upper Carnian to lower Norian) are characterized by abundant micrite-cemented crayfish burrows, Scoyenia trace fossils, and numerous reduction spots and haloes indicating wet, but not hydric, soil conditions. Paleosols in fluvial-lacustrine sandstone-shale fades of Lithofacies Association IIb (lower Norian) record deposition in semiarid to subhumid climate characterized by seasonal soil-moisture deficits and high evapotranspiration. Pedogenic carbonate in both paleosols includes micritic rhizolith, nodule and burrow-fill morphologies. The carbon-isotope compositions of pedogenic carbonate differ significantly between paleosols within the two lithofacies, indicating different controls imposed by the pedogenic environments. The δ 13 C values of pedogenic, micritic rhizoliths in the paleosols suggest somewhat elevated pCO 2 during the late Carnian-early Norian, two to three times modern levels, and support general estimates of pCO 2 based on similar carbonates in time-equivalent paleosols.

Abstract We recognize a causal association between volcanism and lacustrine limestone precipitation in Newark Supergroup strata. Two other associations are apparent: limestone with vegetation, and limestone interlaminated with siliciclastic beds. These associations reflect the different origins of the limestone and relate to basinal hydrological parameters. Hettangian-age limestone deposits reflect an increase in carbonate production and increased rates of groundwater circulation due to the high geothermal heat flow that accompanied volcanism and rifting. Most meter-scale limestone deposits stratigraphically overlie basalt and have covarying stable-isotope compositions, indicating closed-basin conditions. Limestones with spongy to framework texture, interpreted as tufa deposits, reveal sites of sublacustrine or nearshore springs or seeps. Tufa δ 18 0 values are consistently depleted relative to meter-scale carbonates from the same basin, indicating higher temperature waters at debouching localities. Strong climatic seasonality is inferred from carbonate turbidites and euhedral calcite laminae interbedded with siliciclastic deposits. The turbidites formed when rains flushed littoral-zone carbonates into profundal depths, and euhedral calcites reflect reducing conditions in stratified lake waters associated with dry periods. Paleoclimatic differences north to south along the rift basin trend are confirmed by a 5‰ difference in δ 18 O values of meter-scale limestones. The northern Hartford and Fundy Basins have enriched values, indicating increasing aridity northward, with a resulting increase in evaporation from those rift lakes. Littoral-zone carbonates associated with plants and stromatolites contain 13 C-depleted carbon isotope signatures, reflecting organic mediation in carbonate precipitation. In contrast, profundal-zone carbonates have more uniform δ 13 C values, reflecting a greater degTee of equilibration between the atmosphere and the lake water. Newark Supergroup rift-basin limestones reflect a combination of environmental and climatic factors that increased the dissolved-load and/or bedload transportation of calcite, coupled with mechanisms such as basalt mantling, turbidite deposition, in situ precipitation, and vegetative associations, which enhanced carbonate deposition. Stable-isotope compositions help define these varied hydrogeochemical and paleoenvironmental conditions.

Abstract The New Oxford Formation, consisting of alluvial fan conglomerates, fluvial sandstones, overbank mudstones and carbonate lacustrine strata, extends along the southern margin of the Triassic Gettysburg Basin, Vertisols, calcretes and alpha-type microfabrics within carbonate nodules indicate that the paleoclimate was semi-arid throughout Late Triassic time; however, the presence of lake deposits suggests a period of increased humidity. Alternatively, tectonically-driven subsidence within the rift basin could have affected fluvial drainage patterns, causing lakes to form in topographic lows. The sensitivity of carbonate precipitation in lakes to ground and surface water chemistry and to the amount of detrital clastic input permits discrimination between the role of tectonics and climate in governing lake formation. Two distinct carbonate lakes, 30 km apart, formed in different structural settings along the Gettysburg Basin’s southern margin. The lacustrine sediments near Rheems, PA were deposited in a shallow, oligotrophic, palustrine setting associated with perennial flood-plain saturation. In contrast, lacustrine deposits from near Thomasville, PA formed in a deep, episodically stratified or meromictic lake. The sediments record initial flooding over a coarse-grained basal conglomerate, followed by an upward deepening trend, that reversed to a shallowing-upward trend, ending in subaerial exposure and fluvial deposition. Together, lacustrine and associated facies indicate that fault-controlled, localized differential subsidence was responsible for creating the accommodation space required for lacustr ine deposition and that the climate remained semi-arid throughout late Carnian-early Norian time.