The Effect of Basin Subsidence and Climate Change on Acid Sulfate Paleosols Within the Cretaceous (Berriasian 137-144 Ma) Purbeck Group, Dorset, England.
Dennis Terry Jr. and Edwin Anderson. Temple Univ, Dept of Geology, 303 Beury Hall, 1901 N. 13th Street, Philadelphia, PA 19122
Paleosols are preserved in ancient marginal marine to freshwater facies of the Lower Cretaceous (Berriasian: 137-144 Ma) Purbeck Group in Dorset, England. The paleosols developed on clays and shales within a mixed carbonate/siliciclastic system and display both lateral and vertical variability as a function of basin subsidence and climate change. A progressive change up-section from well drained to poorly drained paleosols suggests increasingly humid conditions during the Berriasian. We have documented 36 paleosol profiles across the basin at high, intermediate, and low subsidence localities. These profiles represent approximately 30% of the potential paleosol profiles in the Dorset coastal sections of the Purbeck Group. In outcrop, the top horizons of individual profiles are commonly dark gray to black clay, but in some instances are dark blackish green or mottled with a combination of blue, green, black, and dark gray colors. In some cases these upper horizons contain a slightly angular to subangular blocky structure resembling peds. Horizons lower in the profiles are commonly either a dark reddish black or black. Relict bedding increases downwards in all profiles, commonly ending in shale. In some profiles relict bedding is manifested as a distinct layer of shell debris, where in others the amount of shell debris increases toward the base of the profile. The tops of individual profiles contain abundant yellow (jarosite) to yellowish orange (jarosite and iron oxide) sub-horizontal to sub-vertical root traces. These traces gradually decrease downward. This same yellow-orange staining is seen along fractures within the profile, and also decreases with depth. Nodular siderite, unidentified fossil bone fragments and teeth, and coalified remains of woody material are common in these profiles. Many of the same features seen in outcrop are also visible in thin section. Roots are preserved in the upper part of the profiles as jarosite stained traces with iron oxide and euhedral gypsum, and as pyrite-filled traces in the lower parts of the profile. Soil structure is occasionally manifested as weak angular to subangular blocky peds in the upper part of profiles, but decreases with depth in favor of relict bedding. Organics are preserved as both in situ and detrital remains. In situ remains are commonly coalified, but also exist as occasional reddish brown fibrous masses with original cellular structure. Detrital fragments consist of fibrous, coalified, and charcoal fragments. Some shell material was replaced by pyrite. Although soil microfabric is commonly described in paleosol studies, these profiles have been subjected to structural deformation with slip accommodated along clay-rich horizons. Deformation has thus produced a chaotic microfabric in which tectonic fabrics have overprinted original soil fabrics, making interpretations based on microfabrics suspect. The jarosite staining along root traces and fractures, grayish black, blue, and green coloration, abundant plant material, absence of leaching, and abundant relict bedding all suggest that these paleosols formed as the result of water-table fluctuations, likely within ancient marginal marine brackish to freshwater wetlands and estuarine environments. During periods of higher relative water tables, organic matter accumulated in acidic marshy/swampy settings. Organic decay was anaerobic, as evidence by the preservation of some root traces as pyrite framboids. With the fall of the water table, the soils became exposed, increasing their drainage, and oxidizing the upper parts of the profiles. Roots preserved as pyrite were converted to yellowish jarosite as an oxidation byproduct. This same yellowish staining developed along fractures in the soil. The downward limit of jarosite staining in the paleosols likely reflects the depth to which the water table dropped before subsequent flooding during the next depositional event. Root traces in thin section below the oxidized jarosite zones still show the original pyrite infills of root traces.At least five unique paleosols can be recognized within these profiles. Each represents a distinct paleoenvironment of soil formation along these ancient coastal landscapes. In some instances, the pedogenesis is compound, with two separate paleosols recognized on the basis of individual jarosite-stained root zones separated by an interval of relict bedding. Within modern classification schemes these soils would fall into the Histosol, Inceptisol, and Entisol soil orders of USDA Soil Taxonomy.Given the presence of jarosite, aquic features and abundant plant remains, these paleosols would be classified as sulfahemists, sulfaquepts, and sulfaquents. A progressive change up-section from well drained to poorly drained paleosols suggests increasingly humid conditions during the Berriasian.