Isotopic Imprint of Climate and Hydrogeochemistry on Terrestrial Strata of the Triassic-Jurassic Hartford and Fundy Rift Basins

Abstract

ABSTRACT Late Triassic to Early Jurassic terrestrial sequences in the Hartford and Fundy rift basins have distinctive carbon and oxygen isotopic compositions of calcite and dolomite. The isotopic data mostly reflect paleoclimatic fluctuations and hydrogeochemistry of the lacustrine, playa, and fluvial environments. Dolomites from laminae in three sequences of playa red mudstones and lacustrine gray to black mudstones in the Hartford basin have variable isotopic compositions (13C = -5.8 to + 1.8 PDB; 18O = -7.2 to +0.7 PDB). Within any single symmetrical cycle of playa red mudstone--lacustrine gray, black, gray mudstone--playa red mudstone, there is a systematic change to relatively enriched 13C compositions in dolomite in the grayish black and black mudstones in the center of the cycle. These carbon isotopic data suggest that the lacustrine sequences formed as the lakes changed from well mixed with anoxic bottom waters to stratified with anoxic bottom waters where 13C-depleted carbon was concentrated in organic matter that was then buried. Calcites from lacustrine, micritic, and biomicritic limestones of the Scots Bay Formation of the Fundy basin have stable isotopic compositions (13C = -4.6 to -2.2 PDB; 18O = -6.1 to -3.0 PDB) that become more enriched in 18O and 13C upward in shallowing depositional sequences. These isotopic data reflect initial calcite precipitation when a high inflow of fresh water produced high lake levels, followed by progressively lower inflow, resulting in lower lake stands and higher salinity due to continuing evaporative loss of surface wat r. The lake waters were well oxygenated at all times. In the Hartford basin, caliche calcites in fluvial mudstones and sandstones have isotopic compositions (13C = -7.3 to -3.8 PDB; 18O = -8.0 to - 5.6 PDB) that reflect paleosol processes during climatic conditions that varied from warm and dry in Late Triassic time to relatively cooler and probably wetter in the Early Jurassic. Isotopic compositions of caliche calcites in redbeds in the Fundy basin indicate a parallel climate change from Late Triassic to Early Jurassic time, but also that the climate was relatively hotter and probably drier over the entir interval, as compared to the Hartford basin.