TIME-SPACE DISTRIBUTION OF A PECULIAR CAMBRO-ORDOVICIAN THROMBOLITE: MICROBIALITE BIOGEOGRAPHY?

A peculiar branching, hedgerow maze-like thrombolite is restricted to the Marjuman to Skullrockian of the Cambro-Ordovician of Laurentia. However, within this timeframe, the thrombolite’s distribution suggests a diachronous migration around the Laurentian margin. This thrombolite is distinguished by the maze-like organization of the ridge- or garden wall (maceria)-like construction in plan-view, the relative consistency of maceria width (approximately 1 cm), and the polymorphic nature of the dark mesoclots (2-4 mm in diameter).

Beginning in the early Late Cambrian (Marjuman), the first occurrence of this microbialite is in the Canadian Rockies, in the Sullivan and Waterfowl Formations. The microbialite also occurs in the younger Bison Creek Formation (Sunwaptan). By the Sunwaptan, the microbialite is ubiquitous in the Great Basin where it serves as a stratigraphic marker in middle and inner ramp deposits (Nopah or Notch Peak Formations). During this time, the microbialite is also found in the Upper Mississippi Valley (Potosi Formation), the southern Appalachians (Gatesburg and Conococheague Formations), and the Argentine Precordillera (La Flecha Formation). By the early Ordovician (Skullrockian), the microbialite is found in the northern Appalachians of the United States (Great Meadows Formation). The youngest occurrences are in Newfoundland (St. George Group; Skullrockian-Tulean) and the Canadian Arctic (various formations; Skullrockian-Stairsian).

Coeval, shallow, warm-water carbonate deposits of other cratons, such as Siberia, South China, and Korea, do not share similar thrombolites, though they are rich in microbialites (other thrombolites as well as stromatolites). Thus, the distribution of this thrombolite suggests biogeographic restriction to Laurentia and biostratigraphic restriction to the Marjuman to Skullrockian.