A TROPICAL CARBONATE RAMP BETWEEN SNOWBALL GLACIATIONS: THE LATE NEOPROTEROZOIC KEELE FORMATION, MACKENZIE MOUNTAINS, NORTHWEST TERRITORIES

The Keele Formation (300-600m thick) lies stratigraphically between Sturtian- and Marinoan-age glacial diamict units of presumed global extent. An informal member, the Keele Clastic Wedge, comprises roughly the upper third of the Keele Formation, and is made up of terrigenous clastic rocks deposited mainly in nearshore marine environments. Most strata of the Keele proper are marine carbonates, with local fluvial terrigenous clastic rocks. Carbonate strata are "Bahamian" in character and similar to Phanerozoic warm-water carbonates in that they contain abundant ooids and peloids of original aragonitic mineralogy and have well-developed muddy peritidal facies. Further support for a tropical origin is provided by paleomagnetic data, which suggest that the Keele Formation was deposited at near-equatorial latitude. Stromatolites in Keele rocks are limited to rare, isolated biostromes, which is somewhat unusual in a Proterozoic succession. Keele facies span a broad spectrum of depositional environments, from deep-slope to peritidal to continental, which are represented across a narrow paleogeographic belt. Lateral arrangement of facies, and the gradual stratigraphic transitions between them, suggest deposition in a ramp setting, as opposed to a rimmed or flat-topped shelf system. Fluvial facies are concentrated at a single locality, indicating that terrigenous clastic sediment was derived from a point source or sources. Slump structures and debrites in deep-water facies imply that the ramp was distally steepened. Close interbedding of deep- and shallow-water facies in Keele rocks suggests rapid, high-amplitude sea-level change, and thus is characteristic of successions deposited during climatic icehouse periods. This study of the Keele Formation indicates that ocean-surface temperatures, at least at tropical latitudes, warmed dramatically between snowball episodes, regardless of what mechanisms were responsible for generating global glaciation.