--> ABSTRACT: Sand-Flat/Playa Mud-Flat-Lacustrine Cycles in Fundy Rift Basin (Triassic-Jurassic), Nova Scotia: Implications for Climatic and Tectonic Controls, by Karl A. Mertz, Jr. and John F. Hubert; #91022 (1989)

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Sand-Flat/Playa Mud-Flat-Lacustrine Cycles in Fundy Rift Basin (Triassic-Jurassic), Nova Scotia: Implications for Climatic and Tectonic Controls

Karl A. Mertz, Jr., John F. Hubert

Blomidon Formation red beds comprise over 200 m-scale cycles of (1) sand-flat sandstone (distal alluvial-fan deposits) and (2) playa sandy mudstone and/or lacustrine claystones. Rift basin subsidence and local sagging along the Glooscap fault system generated sand-flat/playa mud-flat cycles by shifting loci of active fan sedimentation toward and away from the playa surface as fan lobes migrated toward topographic lows.

Episodes of intense aridity are recorded in the sand-flat and playa mud-flat deposits where amalgamated sheetflood packages are characterized by pervasive evaporite mineralization (principally gypsum) controlled by subsurface evolution of a Ca-SO4-Na-Cl brine. Aridity is further evidenced by significant disruption of sedimentary fabrics beneath evaporite crusts, deep mud cracks, eolian sandstone layers and patches, and precipitation of authigenic calcium and magnesium-rich illite/smectite and analcime. Carbon isotopic data from early formed, low-magnesium calcite cements (pre-gypsum) reflect slightly to moderately elevated subsurface salinities that accompanied initial brine evolution.

During relatively wetter periods, lacustrine platy claystones accumulated in shallow, oxidizing lakes that lapped onto the sand flats. Claystone units lack evaporite minerals and textures, and many units are partially burrowed. Carbon isotopic data from calcite cements are consistently lighter than sand-flat/playa mud-flat calcites and were in equilibrium with relatively fresh subsurface pore waters.

Overall, the longer term climate became progressively less dry from latest Triassic to earliest Jurassic time as evidenced by (1) a distinct upsection increase in the ratio of lacustrine claystone to playa sandy mudstone, (2) decreased degree of evaporite mineralization in progressively thinner sand-flat units, and (3) a general up-section trend toward lighter carbon and oxygen isotopic ratios that suggests progressively decreased subsurface salinities.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.