Testing the relationship between pedofacies and avulsion using Markov analysis

Markov analysis was performed on a suite of stratigraphic sections from the Willwood Formation (lower Eocene) in the McCullough Peaks area of the Bighorn Basin (Wyoming) to test a current depositional model that links paleosol maturity to floodplain architecture. Under this model of alluvial aggradation, fine-grained sediments in alluvial systems are divided into two basic packages - true overbank deposits and avulsion deposits. True overbank deposits form during flooding events as the water level in a well-established channel exceeds the elevation of the levees and inundates the adjacent flood basin. These deposits are thought to be represented by relatively mature cumulative paleosols with minor amounts of intermixed sandstones and immature paleosols. Avulsion deposits are formed during the initial stages of channel reestablishment and are thought to be characterized by immature paleosols intermixed with ribbon and thin sheet sandstones. Predicted stacking patterns of floodplain lithologies under this model show abundant transitions within and between sandstones and low maturity paleosols (deposited during aggradation of the avulsion packages) grading upwards into higher maturity paleosols; (as the new channel becomes established) and ending with an abrupt transition from highly mature paleosols back to a low maturity avulsion. sequence (as another avulsion invades the low lying floodplain). Markov analysis of the stacking pattern of lithologies in the McCullough Peaks area indicates that the observed sequence of facies transitions is non-random and that the transitions that deviate most from random are consistent with the hypothesized relationship between paleosol maturity and avulsion.