LATE ORDOVIDIAN CARBONATES AND PALEOGEOGRAPHY: A COMPARISON OF WESTERN LAURENTIA (GREAT BASIN) AND BALTICA (ESTONIA)

Comparisions of Late Ordovician (latest Caradoc-Ashgill) sections of western Laurentia (Great Basin) and Baltica (Estonia) reveal similar charbonate ramp profiles and sequence stratigrahic patterns. However, facies and faunal contrasts provide insights about their different paleogeographical settings.

Shallow shelf sections in the Great Basin are typically 150-200 m thick, and contain a typical high-diversity photozoan fauna. High-energy facies consist of oolitic and oncoidal shoals; reefs are absent. Deeper-water sections are thinner (~100 m) and conists of heterozoan facies. Equivalent Estonian sections consist of a similar suite of shallow photozoan facies and deep-water heterozoan facies. Significant facies and faunal contrasts exist: Estonian shelf sections are thinner (~80 m), include coral-stromatoporoid reefs, and contain abundant argillaceous material.

These contrasts reflect different paleogeographical settings. The Great Basin sections accumulated along the western margin of Laurentia in a nearly equatorial latitude. The facies patterns and paleoceanographical position suggest that this was an area of oceanic upwelling. Baltica moved into equatorial latitudes in the Late Ordovician (early Caradoc) and the Estonian sections bordered a cratonic basin. Facies and faunas suggest a normal marine condition, with possible reduction of mid-ramp productivity due to relatively high argillaceous input.