Milankovitch-Scale sequence stratigraphy and stepped forced regressions of the turonian ferron notom deltaic complex, South-Central Utah, USA

摘要

Laterally extensive outcrops in south-central Utah of the Turonian Ferron Notom Delta Complex and its bounding shales contain numerous chronometrically age-dated bentonite beds. The chronometry enables estimation of rates of accommodation and accumulation change. Integration of 56 measured sections and walking and tracing of beds allowed rigorous application of the concept of shoreline trajectory, using the lower bentonites as stratigraphic datums. Geometric stratal relationships along with facies dislocations were used to define high-frequency sequences and systems tracts. Forty-three parasequences were grouped into 18 parasequence sets, five simple sequences, and one sequence set. Parasequence sets show repeated, aggradational-progradational, degradational, progradational-aggradational, and retrogradational stacking patterns, which match the recently developed accommodation succession model. The chronometric analysis shows that the high-frequency sequences are Milankovitch-scale (approximately 100,000 yr), and record sea-level falls of up to 50 meters. The most complete sequence set, sequence set 3, shows clear evidence of a prolonged and stepped forced regression, with two additional higher-frequency sequences (possibly 40 kyr cycles), forming a forced regressive sequence set. This matches previous observations that valley systems, associated with the sequence boundaries, are compound, and also record higher-frequency cut-and-fill cycles. Among the various controlling factors causing cyclicity, such as shoreline autoretreat, delta-lobe switching, and allogenic controls such as climate, tectonics, and eustasy, we hypothesize that glacio-eustasy is the most likely control on the cyclicity of Ferron deposition, overprinted on a tectonically active basin. Gradual build-up of glaciers are hypothesized as a possible explanation of the punctuated downsteps observed in the forced regressive sequence set 3.