Paleokarsts and Paleosols as Indicators of Paleoclimate and Porosity Evolution: A Case Study from the Carboniferous of South Wales
A study of subaerial surfaces in the Carboniferous limestones of South Wales has revealed two paleo-karst associations. The first consists of densely piped and rubbly solution horizons lacking features such as calcrete crusts, rhizocretions, and needle-fiber calcite but with meniscus cements and extensive phreatic blocky calcite cements. The paleokarst morphology is comparable to some present-day humid karsts. The other association shows horizons of less well-developed karst which are associated with calcrete crusts, rhizocretions, and needle-fiber calcite. Thick calcrete horizons also occur indicating formation under a semi-arid climate. These subaerial surfaces are associated with only minor early meteoric cementation.
These sets of features can be directly compared to the Quaternary eolianites of Yucatan. Here the Upper Pleistocene deposits, subaerially exposed under a more arid climate, while exhibiting little early cementation, do contain calcrete crusts, rhizocretions, and needle-fiber calcite. The Holocene equivalents, now exposed under the present more humid climate, exhibit extensive blocky sparry calcite cementation but lack calcrete crusts and their associated features.
The recognition of different suites of subaerial features not only allows the assessment of paleoclimates but also provides a means of predicting the degree of cementation concurrent with each subaerial exposure phase. This may prove a useful tool in reservoir evaluation in carbonates.
KeywordsSubaerial Exposure Oolitic Limestone Carboniferous Limestone Oolitic Grainstone Early Cementation
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