Utilizing sequence stratigraphy to develop a depositional model for Silurian (Niagaran) reefs in the Michigan Basin

ABSTRACT Based upon the construction of a high-resolution sequence stratigraphic framework, this paper interprets the evolution of Niagaran (Silurian) reefs in the Michigan Basin as being characterized by episodic reef growth in response to three distinct, third-order-scale, eustatic sea-level fluctuations. The fluctuations are observed in both the northern and southern reef trends and are interpreted to coincide with Silurian eustatic sea-level fluctuations defined at a global scale. The resulting episodic reef growth model, based upon subsurface core and wireline log analysis, is characterized by at least two orders of stratigraphic cyclicity (probably third and fourth order) that likely formed in response to eustatic sea-level change as well as relative sea-level variations. A detailed sequence stratigraphic analysis of the reefs utilizing facies stacking patterns and identification of key surfaces highlights the punctuated growth of these reefs and provides insight into the lateral and vertical facies variability observed in the subsurface. The sequence hierarchy is manifested by thicker (third- and fourth-order) sequences (tens of meters thick) controlled by globally recognized sea-level changes, and thinner (fifth-order) cycles (few meters thick) driven by relative sea-level variations. Local changes in relative sea level were likely controlled by the combination of higher-frequency eustatic variations along with subsidence and autocyclic mechanisms related to reef growth. The higher-frequency (fourth-order) cyclicity, likely due to eustatic sea-level change, played a major role in controlling the lateral and vertical heterogeneity of reservoir facies in these reefs. Understanding of the growth of these reefs utilizing a modern sequence stratigraphic approach provides new insight into the development of the Niagaran reefs while providing evidence for a complex and episodic depositional model that explains the variability observed in the stratigraphic architecture of these reefs.