About This Item
Share This Item
The Rocknest Formation is an early Proterozoic (1.9 b.y.) westward-facing, rimmed carbonate shelf that evolved from a ramp developed on quartzites of the underlying Odjick Formation. Shelf interior facies are cyclic peritidal dolomites; shelf edge facies are reefal stromatolitic boundstones and intraclast/ooid grainstones, and slope facies are turbidites, slope breccias, and shelf edge breccias. The carbonates are overlain by black shales and graywackes of the Recluse Group.
Facies that represent the shelf edge-to-slope transition of the Rocknest Formation are well exposed for almost 200 km (125 mi) along the strike length of the major north-south trending syncline. At the southern end of the syncline, the Rocknest Formation comprises 230 m (755 ft) of slope deposits on the west limb and 660 m (2,165 ft) of shelf edge deposits on the east limb. The difference in the decompacted thickness of the Recluse Group on both sides of the syncline, as measured from the top of the Rocknest Formation to a marker bed in the Recluse, is 590 m (1,935 ft), which represents the total paleobathymetric relief. However, the difference in thickness of the Rocknest Formation, 430 m (1,410 ft), is less than the total paleobathymetric relief, which indicates that most, but not a l of the total paleobathymetric relief was produced by constructional aggradation of the shelf edge. The remaining relief, 160 m (525 ft), was inherited from a preexisting slope at the top of the Odjick Formation and therefore represents the initial relief of the Rocknest slope.
Average inclinations of the Rocknest slope can be calculated given the distance between sections (approximately 4,500 m, 15,000 ft). The initial average inclination of the slope is 2°, and the final average inclination of the slope is 7°.
The sedimentologic and stratigraphic evidence strongly suggests a depositional model involving a ramp that evolves into a steep, marginal escarpment passing seaward into a shallow-dipping slope. As such, the geomorphic development of the Rocknest slope is considered to be analogous to the slopes of many Phanerozoic rimmed shelves, implying that similar evolutionary processes were operative during the early Proterozoic.
End_of_Article - Last_Page 475------------