Limestone Layers and the Flood

Crinoids are echinoderms (related to starfish, sea cucumbers and such.) In fact if you get some especially good fossil criniod fragments you can see some that have the 5-starred shape in the stalk. There were many crinoids around during the paleozoic period and Crinoids are still around today in fact, though not as abundant. Coccoliths are the calcified plates of Protists. If you look here you can see a nice picture of one of those calcified shells. How do they know? Of course scientists look through plates of the various chalks and limestones to distinguish what animals are there and are trained and through experience can distinguish the different types of animals. Though the Coccolithophorids went through a mass extinction there are still some genera around today.This message has been edited by DBlevins, 03-20-2006 11:52 PM

The history of the earth is filled with transgressions and regressions of sea's over the continents. During high sea levels and long periods of time you can have a build up of limestone, expecially in warm shallow areas such as coral reefs. When sea level slowly drops it can leave behind shallow seas blocked from mixing with the main body of water and as the "inland" sea evaporates it becomes more briny. This briny sea can still percolate through the limestone and the Magnesium in this briny sea can start replacing some of the Calcium ions in the limestone. (as a side note, there is an area in Michigan iirc where such a large inland sea occured and we can see this in the rock record. Also in Texas, thus the high oil abundance there.) This chemical replacement of ions forms dolomite which is basically limestone, just that some of the calcium ions have been replaced with magnesium.Maybe this might help here. If you scroll through the powerpoints to page 12 to 17 you will see what I am talking about. Especially the scenario I talked about above a picture is on page 16.

To form dolomite by replacement
of calcite or aragonite (in the limestone) you need:” Water of the right composition and
” A mechanism to move that water through
the limestone:
” Requires periodic flooding of an exposed tidal
flat or “sabkha” over a limestone
” Evaporation that causes evaporites (especially
gypsum, CaSO42H2O) to precipitate
” Two effects:
- increased density of brine, so it sinks through the
limestone
- increase in the Mg/Ca ratio of brineas the Ca-depleted (Mg-rich) brine moves
through the limestone [Cc (CaCO3)] is replaced by
Dolomite (CaMg(CO3)2)This mechanism is the one I outlined in This post.Also, if you might read the link I provided in that same post (the part I'm asking you to read is short and sweet) you might get a better picture of what I was talking about. hint...there are some pictures describing the event in question on page 16 of the link.