BLEACHED SANDSTONES OF THE TRIASSIC RED BEDS OF THE HARTFORD BASIN, CONNECTICUT

In the ~2.4 km New Haven Arkose of the Hartford Basin, bleached-white arkose sandstone units are rarely observed and poorly described. A recently exposed (<18 m thick) outcrop in southern Connecticut contains several thick bleached beds. At the end of the outcrop, a 14 m-wide area of disturbed strata is adjacent to a small igneous intrusion that is likely related to the nearby large Sleeping Giant diabase sill. The contorted nature of the strata indicates that the intrusion was emplaced into wet, unconsolidated sediment.

In the outcrop bleaching is recognized in 3 manifestations: whole bed, partial (gradational) and patchy. For whole-bed bleaching the entire bed was bleached white. Gradational bleaching occurs in fining-upward beds, where white coloration decreases upward, along with increasing mud content. Patchy bleaching occurs in very poorly sorted sandstone, where partial bleaching resulted in pink color and/or red and white splotches. Bleaching is best developed at the base of the outcrop, where a >3m thick, laterally continuous unit of poorly sorted, pebbly, lithic arkose sandstone has been completely bleached. Partial and patchy bleaching is observed in overlying (red) beds of very poorly sorted, muddy sandstone and fine-grained deposits.

Petrographic analyses show that white arkoses have significant feldspar dissolution and recrystallization of detrital mud. Feldspar dissolution is present in lesser amounts and clay recrystallization is absent in red arkoses. XRD analyses identify kaolinite and illite clays in both red and white arkoses. XRF analyses show only minor differences in major and minor elements.

Field and petrographic observations demonstrate that, grain size, sorting and amount of mud matrix are the three main factors that control bleaching. Permeable coarse units with large clasts and low proportions of matrix are preferentially bleached. It is likely that CO₂-charged reducing fluids from the intrusion infiltrated surrounding permeable sediments, dissolved labile components, recrystallized detrital clay, and reduced the iron, bleaching the units. Fluid inclusion microthermometry of calcite located in the disturbed zone shows that fluids ranged in salinity .212 to .061 NaCl equiv. wt. %, indicating low salinity (meteoric?) water and elevated temperatures ranging from 126 to 156 °C.