Sedimentation in Michigan basin during earliest Salina: evidence for an excursion from eustacy
A basal A-1 evaporite member of the Upper Silurian Salina Group, a marine evaporite sequence, appears to record a time of lowered sea level in the Michigan basin. Indicators of shallow sabkha deposition, such as nodular anhydrite and nonplanar stromatolites, occur in parts of the A-1 evaporite. This unit also contains scattered nodules of borate, a nonmarine evaporite, and the bittern salt sylvite. The petrologic evidence for lowered sea level is supported by studies showing that Middle Silurian reefal buildups within the Michigan basin were subaerially exposed during or immediately prior to A-1 evaporite deposition. However, other studies concluded that carbonate platforms in Indiana and Illinois continued to host flourishing Silurian reef communities throughout A-1 evaporite deposition, under conditions of high global sea levels. The conflicting evidence can be resolved only by postulating that the Michigan basin became isolated from surrounding epeiric seas during A-1 evaporite deposition. Tidal and peritidal carbonates - the Greenfield, Limberlost, Sugar Run, and Engadine dolomites - were deposited at the edge of the Michigan basin during this time. These units could have served both as sills and as shorelines that separated open epeiric seas from a partly desiccated basin. Evaporating brines, pooled below sea level in the basin, could have been replenished by seawater and meteoric water flowing through shallow inlets or seeping into the basin through the subsurface. Basin isolation may have been enhanced by minor eustatic fluctuations in sea level and by the constraints on shallow-water circulation in epeiric seas. Therefore, in analogy to the Miocene desiccation of the Mediterranean, the large apparent drop in sea level recorded by the A-1 evaporite would have been a local rather than a eustatic change.
- Research Organization:
- State Univ. of New York, Stony Brook
- OSTI ID:
- 7041797
- Report Number(s):
- CONF-8610158-
- Journal Information:
- Am. Assoc. Pet. Geol., Bull.; (United States), Vol. 70:8; Conference: AAPG Eastern section meeting, Ann Arbor, MI, USA, 1 Oct 1986
- Country of Publication:
- United States
- Language:
- English
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