HOLOCENE SEDIMENTATION IN TWO WESTERN MICHIGAN ESTUARIES

Changing levels of the Laurentide Great Lakes over the past 14.5 ka have strongly influenced development of surrounding coasts. One of the most striking geomorphic features inherited from earlier levels is the presence of several dozen coast-normal flooded river valleys that occur along the eastern margin of the lake basin. Westward flowing rivers deeply cut through preexisting terraced fluvial deposits and Pleistocene glacial outwash during the Chippewa lowstands between 10 and 8 ka BP. Fresh water estuaries were formed as these valleys were flooded during the post-late Chippewa transgression, and as rivers graded their fluvial terraces to Nipissing levels. More recent (~5 ka BP) attainment of somewhat lower lake levels has been accompanied by regressive progradation of most estuarine deltas. Two freshwater estuaries, Manistee Lake and Pentwater Lake, in northwest Michigan were examined in detail. Both are partially filled with post-late Chippewa sediment comprising terrigenous and organic fluvial, deltaic, and estuary-center facies that comprise an almost complete record of Holocene sedimentation in these settings. Stratigraphic successions in either estuary consist of a transgressive-regressive fluvial-deltaic-estuarine sequence deposited over the past 10,000 years. A typical section consists of basal fluvial-deltaic sand, thick (>20 m) transgressive dark brown lake mud (gyttja), regressive interbedded prodelta sand and mud often containing abundant pebble to cobble size wood debris, and highstand fluvial-deltaic sand of modern deltas. These sequences represent change in Lake Michigan water level during the post-late Chippewa transgression, the Nipissing highstand and stillstand, and the post-Nipissing regression. Radiocarbon dates of allochthonous organic material in Manistee Lake indicate an abrupt decrease in accumulation of terrigenous components at about 3.1 ka, and relatively invariant sedimentation rates at about 4.5 m/ka since that time.