PETROGRAPHIC ANALYSES OF CORE FROM THE LOWER JURASSIC PORTLAND FORMATION, HARTFORD BASIN, CONNECTICUT

Outcrop exposures of the lower Portland Formation are limited, especially in the central part of in the Hartford rift basin. A series of cores recovered from a E-W transverse just south of Hartford during the development of the Park River tunnel project, represents more than 500 m of stratigraphic section. Primarily mudstone with subordinate relatively thin sandstone beds, analyses of the sedimentary facies indicate deposition in a lacustrine environment— more similar to that of the underlying East Berlin Formation than to that of the fluvial (better exposed) upper part of the Portland Formation.

Sandstones from four cores in the upper 100 m of section and one core in the lowermost part of the section were sampled for petrographic analyses (500 or greater point counts). Samples from all the cores are similar in composition and diagenetic features. Sandstones are feldspathic sandstones (arkose) with average framework grain composition of about 55% quartz, 40% feldspar and 5% rock fragments (primarily granitic high-grade metamorphic and intrusive rocks). Feldspars are primarily plagioclase, but exact percentages are difficult to obtain because of extensive albitization of grains. Feldspars are also highly affected by dissolution, seriticization and replacement by calcite cement.

Other diagenetic processes include cementation and dissolution of grains and cements. Typical cementation (average 22% volume, range 16-31%) includes early hematite dust-rim cementation (1%), followed by nearly synchronous feldspar (6%) and quartz (4%) overgrowths. In several samples extensive overgrowths completely fill pores, otherwise patchy calcite cement (8%) may fill remaining (if any) pore space. Calcite also locally replaces both quartz and feldspar grains and overgrowths. In a few samples late Fe-dolomite (5% max.) and clay (<1%, smectite? or mixed layer) are present. Porosity varies greatly (0-18%, avg. 6%). Most samples contain secondary porosity, with least two stages of formation, pre-calcite and post-calcite emplacement. Hydrocarbon and iron-oxide residues are contained in the later stage pores.