VARIATION IN LACUSTRINE  AND SPRING CARBONATE 
DEPOSITION AND DIAGENESIS: NORTHERN, CENTRAL AND 
SOUTHERN RIFT BASINS, EASTERN NORTH AMERICA

	DE WET, Carol, B., Dept. of Geosciences, Franklin & 	
		Marshall College, Lancaster PA, 17604
	GIERLOWSKI-KORDESCH,  Elizabeth,  Dept. of Geological 
		Sciences,  Ohio University, Athens, OH, 45701-2979
	GORE, Pamela, J.W.,  Geology Dept., 555 North Indian Creek 
		Drive,  Clarkston, GA 30021-2396

Volumetrically, the amount of non-diagenetic carbonate in the 
Mesozoic rift basin lakes is small compared to the amount of 
siliciclastic sediment, but it is important in terms of the information it 
contains.  Understanding the mechanisms responsible for carbonate 
precipitation and deposition in the rift basins provides information 
about biologic, climatic and source terrain changes.  Carbonate 
precipitation occurs in freshwater lakes when pH is affected by plants 
and algae.  Carbonate may precipitate when ion activity in lake water 
changes due to evaporation and increasing salinity. This has often 
been interpreted as indicative of increasing aridity.  When older 
carbonate terrain is weathered, clasts and ions are transported into the 
catchment area and new carbonate beds may accumulate.  In arid to 
semi-arid climates, carbonates also precipitate within the soil, forming 
calcrete nodules and/or layers.
	Within the Newark Supergroup there are three types of 
carbonate systems: (1) localized carbonate deposits, representing 
precipitation from springs (hot and cold) and seeps; (2) episodic 
carbonate-domination in otherwise siliciclastic-dominated, widespread 
lake sediments; and (3) intermediate-sized carbonate accumulations 
that reflect sites of relatively long-lived palustrine or lacustrine 
deposition.  Individual basins contain predominantly one system, 
although there is overlap and the systems are not mutually exclusive.  
For example, hot spring and tufa deposits occur within the Fundy and 
Hartford basins, but the Fundy also contains lacustrine beds that are 
almost exclusively limestone that formed in the third type of system.  
The Newark Basin lakes were predominantly siliciclastic, but episodic 
carbonate-rich turbidites and precipitated- carbonate beds 
characteristic of the second type of system occur.  The Gettysburg 
Basin contains carbonate interpreted as having formed in both the type 
two and three systems. 
	The southern basins contain both type two and three systems, 
where rare occurrences of carbonate overwhelmed the predominantly 
siliciclastic depositional regime.  In the Culpeper Basin, local type two 
carbonate deposition is present in the Triassic part of the section.  
Lacustrine stromatolitic limestone layers are present. In the Jurassic 
part of the section, type three carbonate deposition occurs, for 
example, in the Midland Fish Bed and in the Waterfall Formation 
(where carbonate turbidites are present).
	In the Deep River Basin, the Durham sub-basin is dominated 
by fluvial clastics, but thin layers of type two carbonate are present 
locally.  The Durham sub-basin contains beds of wavy limestone up to 
20 cm thick, which probably represent system two deposition 
(Wheeler and Textoris 1978).  In the Sanford sub-basin, however, 
lacustrine black shales and coals are abundant, and carbonates are 
extremely uncommon, despite the persistence of large deep lakes.  
Calcrete carbonate has been reported from all of the rift basins.
	Age dating is not yet refined enough to see if the carbonates 
correlate in time between basins.  If correlations can be established, 
information about climatic shifts north to south along the Pangean 
break-up margin could be obtained.  In conjunction, by noting when 
and where large accumulations of carbonate were being deposited, the 
timing of faulting and/or amounts of erosion could be estimated as 
carbonate host rocks were breached.
	Diagenesis of the different carbonates also reveals similarities 
and differences between the basin's burial histories and internal 
plumbing systems.  Initial data show that carbonate deposits from 
different basins have distinctive carbon and oxygen isotopic 
signatures, distinctions that were maintained through diagenesis.  
Petrographic, cathodoluminescent and geochemical differences show 
broad overlap, but each system has some unique characteristics.  

Wheeler, W.H., and Textoris, D.A., 1978,  Triassic limestone and 
chert of playa origin in North Carolina: Journal of Sedimentary 
Petrology, v. 48, p. 765-776.