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Abstract:

Differences in stratigraphic organization of three examples of carbonate source-rock formations are compared in terms of three-dimensional architecture and sediment flux. A hypothesis is proposed to explain the observed patterns, involving different orders of relative sea-level fluctuations and climatic change.

All examples were studied in the same systematic way. Carbonate and organic-matter content were measured in sections, which are correlated in a high-resolution time framework (second-order to fifth-order scale sequences) from platform to basin. The examples are the Upper Devonian (Frasnian) Duvernay Formation in western Canada, the Upper Carboniferous (Desmoinesian) Paradox Formation in the western United States, and the Upper Cretaceous (Cenomanian–Turonian) Natih Formation in Oman. Different orders of cyclicity appear to control the distribution of organic-matter in these formations: in the Devonian example the main control is at the second-order scale, in the Carboniferous example the fourth order is dominant, and the Cretaceous example is dominated by the third-order scale.

In order to characterize the sediment flux in these depositional systems, the ratio of the carbonate fraction to the organic-matter and other mineral fractions (clays, silts) was studied. In several examples, an inverse linear relationship was found between the carbonate fraction and the organic-matter plus mineral fraction. With good evidence for the variation in the carbonate production, this leads to the suggestion that the ratio of the organic matter production to the clay input has been essentially constant for considerable periods of time.

We propose that the three different patterns of the carbonate source-rock distribution illustrated in this paper are characteristic of three climatic conditions of the earth system: the icehouse state (the Upper Carboniferous example), the intermediate-house state (the Upper Devonian example), and the greenhouse state (the Upper Cretaceous example). The internal organization of carbonate source rocks would, in that case, be predictive, and directly controlled by the known dominant sequence order in these respective states.

The interpretation of the inverse linear relationship as the result of a varying carbonate flux with stable background sedimentation of the organic-matter and mineral fraction has several implications. It suggests that organic-matter production was, at times, less sensitive to short-term climatic variations than the adjacent carbonate platforms. In addition, the TOC–carbonate plot may be a useful way to characterize and compare carbonate source rocks in space and time. This would be particularly important when a relationship can be demonstrated between the mineral content and the maturation, expulsion, and migration characteristics of these source rocks.

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