LOWER PALEOZOIC OF THE EAST LAURENTIAN CONTINENTAL SLOPE: EUSTATIC/CLIMATE CONTROLS ON MACRO- AND MICROSCALE MUDSTONE ALTERNATIONS

The Taconian allochthons preserve earliest Cambrian rift facies, overlying passive margin slope deposits, and synorogenic rocks of the Middle–Late Ordovician Taconic orogeny. Cambrian–Early Ordovician facies in Taconian NY and Quebec are a thin succession (locally <600 m) dominated by burrowed, weakly–non-calcareous, greenish mudstones. However, up to eight regionally extensive, non-burrowed, macroscale black mudstone alternations with allogenic bedded limestones developed on the middle slope with bottom water dysoxia or anoxia. The oldest dysaerobic intervals (DIs) are the Browns Pond DI (upper Lower Cambrian) and lowest part (terminal Lower Cambrian) of the Hatch Hill DI. Black mudstone-dominated intervals are equated with sea-level rise, climate amelioration, reduced bottom water circulation, and re-establishment/progradation of the carbonate platform (Dunham, Winooski formations, respectively). Intervening green mudstones (local glauconitic quartz arenites in Quebec) are equated with the Hawke Bay regression. The ca. 5 Ma Browns Pond–initial Hatch Hill interval is succeeded by the ca. 20 Ma (latest Early Cambrian–middle Tremadocian) Hatch Hill DI. Sea-level changes in this interval of high sea level and slope bottom water dysoxia may be marked by thick debris flows and turbidite sandstones. Overlying Ordovician greenish mudstones in NY and Quebec show meter-scale “Logan cycles” with sharp lower contacts and upward increase in redox (black-green-buff) and carbonate content. Logan cycles, with a duration of ca. 100 Ka, may be climate forced. Five black mudstone alternations in the Ordovician (middle Tremadocian; early, middle, late, and terminal Arenigian; early Caradocian) roughly correlate with highstands. Passage of the Taconic peripheral bulge and deposition of condensed red muds, radiolarites, and ashes on the slope (e.g., Indian River Fm.) mark onset of the Taconic orogeny.