DIAGENESIS AND THERMAL HISTORY OF THE SANDSTONES IN THE EARLY MESOZOIC DEERFIELD RIFT BASIN, MASSACHUSETTS

In the Deerfield basin, the stratigraphic sequence from oldest to youngest is: 1.7 km fluvial Sugarloaf Arkose; 3-9 m Fall River lacustrine/marsh beds; 50-100 m Deerfield Basalt; 2 km playa/lacustrine Turners Falls Formation; and 300 m alluvial-fan Mount Toby Conglomerate. During burial diagenesis, downward-circulating groundwaters moved through the sands, driven by topographic relief around the basin, and co-precipitated albite and quartz overgrowths as ubiquitous early cements. In the deeper parts of the basin, feldspar grains were partly to completely albitized. In early Jurassic time (~201 Ma) increased extension led to melting of the upper mantle and extrusion of the Deerfield lavas. The high heat flow generated a deep-basinal, hot brine. Rising hydrothermal plumes and in situ hot brine produced four patterns of diagenetic minerals superimposed on the early burial cements: chert-illite-pyrite; illite (stacked plates and fibrous); mosaic albite; and bleached mosaic albite. These patterns were mapped using field observations and 175 sandstone modal analyses. Illite precipitation in the Sugarloaf sandstones was accompanied by widespread dissolution and replacement of sodic plagioclase grains, enriching the brine in sodium. Type-2b authigenic chlorite in the bleached mosaic albite pattern was precipitated at about 200 degrees C. Biomarker data for lacustrine black mudstones show that the basin is in and beyond the "gas window." Argon spectra for five detrital microcline crystals are compatible with gradual cooling of the basin through 150 degrees C at about 170-150 Ma. The spectra also show that the microcline cooled through 300-350 degrees C at 280-240 Ma, confirming Alleghanian metamorphism of lower Paleozoic source rocks east of the basin.