THE RELATIVE MATURITY OF PALEOSOLS (CHRONO AND TOPOSEQUENCES) FORMED WITHIN AN ORBITALLY FORCED HIERARCHY OF CYCLES: THE PURBECK GROUP, LOWER CRETACEOUS, DORSET, ENGLAND

Paleosols formed on 6th, 5th, 4th and 3rd order cycle or sequence boundaries in fresh water to marginal marine facies of the Purbeck Group in Dorset. In this hierarchy 6th order cycles are the product of precession and 5th, 4th and 3rd order sequences represent sets of precessional cycles bundled by varying degrees of eccentricity in the Earth's orbit. In a 20-meter thick interval in the Middle Purbeck at Durlston Bay, weakly developed pedocals formed on the tops of beds 81, 83, 93, 100, 102 and 115 (bed numbers of Clements). These pedogenically-modified surfaces all represent cycle (or sequence) boundaries and the maturity of soils on these surfaces can be directly related to the surfaces' position in the cyclic hierarchy. Soil maturity varies from a concentration of terrestrial mammal fossils in organic rich terrigenous silts (at the low end) to organic-rich shales with well-developed root-mat systems to meter-thick shales with well-developed ped morphology throughout (most mature). The first type occurs at 6th order boundaries, the second variety with root systems was found on 5th and 4th order boundaries and the thick sequence of peds at a 3rd order boundary. This pattern of occurrence represents a kind of chronosequence. A pedogenic toposequence is observed at two cycle boundaries (tops of beds 102 and 115) that can be traced landward (toward the basin margin) from Durlston Bay to the Bonnfield and Sunnydown Farm quarries, a distance of 10 km. When traced in the landward direction correlative soil horizons are thicker and have better developed root-mat systems reflecting longer periods of soil development. Both paleogeographic and stratigraphic position within the allocyclic hierarchy play a role in the degree of pedogenesis in Purbeckian facies.