Galatia Channel:Rapid Transgression, Gradual Regression
Rapid Transgression, Gradual Regression
Evidence from the Pleistocene, particularly the most recent deglaciation, indicates that melting can be surprisingly rapid, even “catastrophic.” Blanchon and Shaw (1995) inferred, based on drowned reefs in the Caribbean and Gulf of Mexico, three “catastrophic, metre-scale sea-level rise events” during the last Pleistocene deglaciation. Gregoire et al. (2012) calculated that a sea-level rise of 45.9 to 59 ft (14 to 18 m) took place within a span of about 350 years close to 14,000 years ago, and a rise of 29.5 ft (9 m) took place within 500 years about 8,200 years ago. Data from Greenland ice cores indicate two remarkably sudden Late Pleistocene warming events. One at 11,700 years B.P. lasted 60 years, and an earlier event at 14,700 years ago spanned a mere 3 years (Steffensen et al. 2008). Other evidence and examples may be found in Webster et al. (2004), Wright et al. (2009), Bird et al. (2010), and Törnqvist and Hijma (2012). In contrast, the growth of a continental ice cap requires a much longer interval of time, measured in thousands to tens of thousands of years (Muhs et al. 2011; Dutton and Lambeck 2012).
The authors cited above suggest that any triggering event that raises the sea level may set off a chain of events leading to the rapid destruction of ice caps. The trigger could be the onset of a warmer climate or the failure of ice dams that hold back large bodies of fresh water, such as glacial Lake Agassiz and Lake Missoula. The rising sea breaks up floating ice sheets and releases fleets of icebergs, which melt in warmer waters and raise the sea level further. Moreover, the loss of sea ice changes the ocean and atmospheric circulation, leading to more melting. In contrast, the growth of continental glaciers is “glacially slow” because snow and ice can accumulate only so fast. In fact, the global cooling required to bring on an “ice age” reduces the capacity of the atmosphere to hold water vapor and yield snow.
The rapid drowning and burial of the Springfield peat swamp has counterparts in other late Paleozoic deposits. Ravinement surfaces are reported from other coals, such as the Herrin Coal of Illinois (DeMaris et al. 1983), which also has a buried autochthonous flora adjacent to the Walshville paleochannel (DiMichele and DeMaris 1987). Lower Permian carbonate cycles of Kansas commonly begin with a thin transgressive lag of fish bones, ostracods, and intraclasts, implying abrupt, erosive transgression (Miller et al. 1996). The rapid marine transgression that terminated peat accumulation repeats throughout the Pennsylvanian in the Illinois Basin among coal beds that lack channels and gray shale roof. Most commonly, the coal is sharply overlain either by offshore black, phosphatic black shale or by marine limestone; at the contact, coal laminations are generally truncated, indicating some erosion of the top of the peat body. At the base of black shale, a thin pyritic shell breccia may be present (Zangerl and Richardson 1963). Where nonmarine gray shale overlies low-sulfur coal, in situ tree stumps and tidal rhythmites are commonly in evidence. We have logged examples including the Lower Block, Murphysboro, Colchester, Herrin, and Danville Coals in addition to the Springfield.
Such conditions of rapid, pulse-like sea-level rises likely also occurred during the Pennsylvanian ice age (Archer et al. 2015, 2016). Such considerations further militate against the notion of “keep up” time-transgressive peat-swamps, created by rising base level and driven across the landscape by sea-level rise (e.g. Heckel 1995). Peat formed much too slowly to keep up with the abrupt sea-level rise of a typical deglaciation.
W. John Nelson, Scott D. Elrick, William A. DiMichele, and Philip R. Ames xxxx, Evolution of a Peat-Contemporaneous Channel: The Galatia Channel, Middle Pennsylvanian, of the Illinois Basin FINISH CITATION