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.
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