Deposition and Geochemistry of Lacustrine and Spring Carbonates in Mesozoic Rift Basins, Eastern North America
Carol B. De Wet, Claudia I. Mora, Pamela J.W. Gore, Elizabeth Gierlowski-Kordesch, Steven J. Cucolo, 2002. "Deposition and Geochemistry of Lacustrine and Spring Carbonates in Mesozoic Rift Basins, Eastern North America", Sedimentation in Continental Rifts, Robin W. Renaut, Gail M. Ashley
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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 δ180 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 δ18O 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 13C-depleted carbon isotope signatures, reflecting organic mediation in carbonate precipitation. In contrast, profundal-zone carbonates have more uniform δ13C 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.
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Continental rift basins have long been of interest to sedimentologists. Of all the terrestrial settings, rift basins typically provide the greatest accommodation space, and consequently have some of the longest records of continental sedimentation. These records are a product of a complex interplay between several factors that include geological structure and tectonic activity, volcanism, climate and its temporal variability, hydrology, biology and time. Sedimentation in Continental Rifts is a timely update on this exciting interdisciplinary field and presents new approaches and insights into tectonic and structural controls of sedimentation. Other topics included are lacustrine and fluviatile depositional environments and some lesser-known settings, such as springs, wetlands, and paleosols. Several papers consider the behavior of silica in rift lakes, particularly the roles of microorganisms in silica precipitation, whereas others examine the paleoenvironmental importance of freshwater carbonates. The contents of the volume show that sedimentological research in rift basins has progressed beyond basic facies description and general models, and is now focused on understanding the integrative effects of physical, chemical and biological processes in rifts.