DISTRIBUTION AND ORIGIN OF CLAY-MINERALS IN THE LOWER JURASSIC MCCOY BROOK FORMATION, MINAS BASIN, NOVA-SCOTIA

The distribution of clay minerals in the depositional facies of the Mc Coy Brook Formation is controlled by the clay-mineral origin (extrabas inal versus intrabasinal). The McCoy Brook Formation was deposited dur ing Early Jurassic (Hettangian-Sinemurian) time in the Minas basin as alluvial-fan, sandflat, playa, lacustrine, fluvial, eolian, debris-flo w and talus facies during a period of semiarid to arid climate. The pr edominant minerals in the clay fraction of the formation are illite, s mectite, chlorite, and palygorskite. These minerals are partitioned in the depositional environments in three assemblages. Assemblage one co mprises dominantly illite with lesser concentrations of smectite, chlo rite and palygorskite. This assemblage characterizes alluvial-fan, san dflat and playa facies. Assemblage two consists predominantly of smect ite, with trace concentrations of illite and chlorite. This assemblage is characteristic of fluvial-channel, eolian, debris-flow and talus f acies. Assemblage three comprises mainly illite, trace amounts of chlo rite, and trace concentrations or no smectite. This assemblage is foun d in fluvial floodplain and lacustrine facies. Illite and chlorite are mainly of extrabasinal origin, having formed from weathering in the s ource area. Smectite and palygorskite are of intrabasinal origin. The smectite, identified as Fe-rich saponite, formed from alteration of ba salt clasts within eolian, debris-flow and talus facies and of basalt outcrops adjacent to fluvial-channel facies. Palygorskite formed durin g pedogenesis in facies exposed at the surface for long intervals duri ng deposition under semiarid to arid conditions.