107-22 Stratigraphic Interpretation of Stromatolites on a Mixed Carbonate-Clastic Platform: The Prairie du Chien Group (Early Ordovician, MN and WI)

Session: Sedimentary Geology Division/SEPM Student Research Poster Competition: Dynamics of Stratigraphy and Sedimentation

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Abstract:

In the Precambrian, stromatolites were diverse and abundant, but by the end of the early Ordovician, stromatolite form diversity had dropped substantially. In the present, stromatolites are rare in marine systems and display a limited range of form. Causes for stromatolite decline are not entirely known but are likely some combination of biological competition, shifting marine water chemistry, and restructuring of carbonate platforms. Stromatolite-bearing platforms of the early Paleozoic provide an opportunity to examine the relationship between environmental restriction and morphological diversity. The Shakopee Formation of the Prairie du Chien Group (early Ordovician) contains stromatolites in a mixed carbonate-clastic system, permitting examination of the stratigraphic position of stromatolites in the context of clastic supply and sea level change. The boundary between the lower and upper members of the Shakopee has been identified as a sequence boundary that represents a significant change in environment, but the position of a prominent stromatolite horizon with respect to this boundary is uncertain. In this study, we examine whether the mid-Shakopee stromatolite horizon represents deposition in very shallow water immediately below the sequence boundary or development during initial transgression above the sequence boundary. The first hypothesis is suggested by comparison with modern marine analogs, while the second utilizes stratigraphic patterns commonly observed in Precambrian successions. We conducted fieldwork in southeast Minnesota and northwest Wisconsin to identify the stratigraphic patterns and common facies in the Shakopee Formation and connect this stratigraphy to the sequence-stratigraphic framework previously identified in eastern Wisconsin. The mid-Shakopee stromatolite horizon rests on shallow, medium- to coarse-grained quartz arenite and records an abrupt loss of clastic influx to the basin. The mixed carbonate-clastic facies following the stromatolite horizon show an increase in proportion and grain size of clastics. This pattern suggests that the stromatolite horizon signals a change towards deeper water and a clastic-starved platform, like many Precambrian successions, where stromatolites develop on flooding surfaces. Additionally, stromatolites have higher synoptic relief at the base of the stromatolite interval, becoming stratiform upward. This change in morphology, not well described in previous studies of the Prairie du Chien, is consistent with the filling of accommodation during bioherm growth. These early Paleozoic stromatolites have previously unrecognized morphological diversity and, like their Precambrian counterparts, are potentially useful in reconstructing stratigraphic architecture.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9325

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