From: bivalve (bivalve@mail.davidson.alumlink.com)
Date: Thu Apr 17 2003 - 17:25:50 EDT
>That brings up another question I have. When I was in school some 35 years ago, there was not a good explanation for the deposition of massive limestones. What are the conditions favorable for deposition of marine limestones?<
The basic factors needed are a good source of calcium carbonate and limited clastic input. Many carbonate sources are biological, e.g. various microfossils, algae, coral, mollusks, echinoderms, or breakdown products of these. In very deep water (1000s of meters), the calcite compensation depth becomes an issue; deeper waters are generally undersaturated with calcium carbonate and dissolve carbonates, so deposition must outpace dissolution to get formation of deepwater limestone. Terrestrial erosion is the main source of clastic material, so areas with coastal mountains (unless the mountains are limestone or marble) and areas near major rivers are often bad for limestone accumulation. Offshore shallow banks like the Bahamas are excellent for limestone accumulation. There are all sorts of sedimentological, paleontological, etc. features that one can look at to get specific ideas about the setting. I am not certain offhand what constitutes a massive limestone versus non-m!
assive; there are some limestones relatively devoid of features, but as this generally includes a fossil shortage, I have not dealt much with them. I suspect many of these are Precambrian or Paleozoic deepwater limestones, as many of the major pelagic fossil groups did not appear until the Mesozoic.
>What might have caused the waters to slosh or rush back and forth like tides?<
>Don't know.<
Without a coherent model, the claim that a YEC model is better is not very plausible. For example, some evidence that a YEC flood would produce sheet erosion would help your claim that paraconformities fit a YEC model. Conversely, claiming that the Flood produced whatever kind of erosion happens to be observed in the geologic record would show that the Flood is a magic wand, unconstrained by physical reality. (I am not questioning the claim that, because one believes certain layers to be deposited in the Flood, therefore whatever erosion they show must have been done by the Flood; rather, it is the claim that the pattern fits the expectation of a Flood model no matter what the pattern may be, because the Flood model in question is unconstrained by physics, geology, logic, or other factors that might give it definition.)
Note also that physical appearance is only one of many possible indicators of an unconformity. Abrupt changes in the type or content of the rocks could indicate an unconformity (e.g., sandstone versus limestone, change in fossil types, change in rock chemistry, change in radiometric dates, or features of the surface such as epibionts or borings).
At the local scale in the US Atlantic coastal plain (where most of my fieldwork has been), unconformable contacts may appear fairly flat, but on the regional scale extensive irregularity is evident, reflecting the complex history of erosion and deposition with fluctuating sea levels. Surfaces show features such as chemical alteration, encrusting organisms (oysters, bryozoans, etc.), boreholes (such as from lithophagid mussels), abrupt changes in lithology and color, abrupt changes in faunal composition (macro and micro), and presence of pebbles and bone lags. All of these are evidence of long time intervals of erosion and exposure and thus pose problems for Flood geology.
Dr. David Campbell
Old Seashells
University of Alabama
Biodiversity & Systematics
Dept. Biological Sciences
Box 870345
Tuscaloosa, AL 35487-0345 USA
bivalve@mail.davidson.alumlink.com
That is Uncle Joe, taken in the masonic regalia of a Grand Exalted Periwinkle of the Mystic Order of Whelks-P.G. Wodehouse, Romance at Droitgate Spa
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