COMPARISION OF THE MONKTON AND ALTONA FORMATIONS (LATEST EARLY-MIDDLE CAMBRIAN): INSIGHTS ON THE PALEOGEOGRAPHY OF THE IAPETUS SHELF

Identification of the Olenellus-bearing horizons in the Altona Fm. in northern NY by Landing et al. (2009) makes this unit the oldest within the Potsdam Grp. It also presents the possibility of the comparing the stratigraphy and depositional environments of the Altona with coeval rocks elsewhere along the Iapetus margin, in particular, the Monkton Fm. in western VT. Because the Monkton Fm in VT lies on the upper plate of the Champlain Thrust it has been transported westward approximately 80-100km (Stanley, 1987). Thus, this heterolithic wave and tidally-influenced deposit on the Iapetus margin was originally deposited further basinward, or east. Facies transitions parallel to strike (S to N) in the Monkton reflects basinward progradation of peritidal facies (Rahmanian, 1981), however there is significant paleoenvironmental variation in the lower Monkton, which exhibits both fluvial, tidal, and wave-influenced features. In contrast, the Altona Fm., located in northern NY, is an autochthonous heterolithic sedimentary unit representing a dominantly wave reworked shelf sediment deposit, often under storm conditions. Although both contain an Olenellus-age fauna and sequence stratigraphy analysis confirms a partial age equivalency of the two units, the Monkton is thicker than the Altona (~400m vs. 84m) and it overlies nearly 0.5km of older Cambrian sediments (Cheshire and Dunham Formations) versus onlapping Precambrian basement. This suggests an earlier onlap of the Laurentian margin from the Sauk sequence sea level rise in VT than in northern NY. Even accounting for differences in age of onlap, the differences between processes recorded in the two units suggests that the coastline of the Laurentian continental margin - Ottawa-Bonnechere aulacogen was significantly embayed. Such paleogeography could explain how the predominantly lower energy Monkton could accumulate further basinward in the Iapetus Ocean, but in a barred coastline setting, while the Altona shelf was exposed and susceptible to Iapetus Ocean wave action.