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The Middle Ordovician St. George Unconformity is the Sauk/Tippecanoe sequence boundary in western Newfoundland. It is a karst unconformity to disconformity to paraconformity. It formed by uplift and erosion during lithospheric flexure as a forebulge passed across the apex of the St. Lawrence Promontory of Laurentia's continental shelf in the initial stages of the Taconic Orogeny. Erosion was preceded by gentle uplift, warping, and block faulting of platform carbonates. Early faults directed groundwater flow and dissolution and locally resulted in subsidence dolines that were sites of anomalously thick pre- unconformity peritidal carbonates. Regional erosion that followed during Whiterockian time probably lasted 1 to 3 m.y. and locally removed as much as 50 m of stratigraphy on blockfaulted topographic highs. Surface karst marked the unconformity with minor karren and proto-soils. Subsurface karst as much as 120 m below the unconformity was manifested as near-surface porosity and sediment-filled fissures, and deeper, structurally controlled, small and large caves filled by dolomite muds, chert sands, and rock-matrix breccias. Upward stoping during breccia formation finally produced marine and nonmarine, sediment-filled collapse dolines at the unconformity. Chert-pebble conglomerate lags rested upon the unconformity before it was onlapped and buried by widespread peritidal carbonates. Faulting active throughout late Ibexian and Whiterockian time is consistent with the passage of the forebulge through western Newfoundland. This and changing rates of subsidence influenced not only the shape of the unconformity, but also pre- and post-unconformity sedimentation, that is, facies, thickness, and cyclic sedimentation; the ordered timing of events upon the shelf and in coeval slope sediments; and a diachronous event stratigraphy from Newfoundland to Quebec. This indicates that (1) early Taconic orogenesis rather than eustatic events fashioned the unconformity and associated stratigraphy and (2) the transformation from a passive to a convergent plate boundary began in the middle Ibexian, long before the unconformity developed. During the early stages of this transformation, imminent convergence of the margin caused the slowing to cessation of subsidence. Consequently, there was abrupt shallowing of the shelf reflected in the sudden change from open to restricted shelf facies throughout western Newfoundland. The model predicts that the timing and duration of events, and the longevity of the unconformity reflects the area's location at the St. Lawrence Promontory. In this respect, western Newfoundland will be different from other areas along the Appalachian system, especially reentrants. The broad scheme of events is consistent, however, from area to area along the length of the margin. |
