Rhythmic Sedimentation in a Mid-Pennsylvanian Delta-Front Succession, Magoffin Member (Four Corners Formation, Breathitt Group), Eastern Kentucky: A Near-Complete Record of Daily, Semi-Monthly, and Monthly Tidal Periodicities
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Rhonda M. Adklns, Kenneth A. Eriksson, 1998. "Rhythmic Sedimentation in a Mid-Pennsylvanian Delta-Front Succession, Magoffin Member (Four Corners Formation, Breathitt Group), Eastern Kentucky: A Near-Complete Record of Daily, Semi-Monthly, and Monthly Tidal Periodicities", Tidalites: Processes & Products, Clark R. Alexander, Richard A. Davis, Vernon J. Henry
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The Magoffin Member of the Four Corners Formation (Breathitt Group) outcrops in eastern Kentucky as a predominantly coarsening-upward succession of rhythmically interstratified sandstone, siltstone, and mudstone with minor occurrences of limestone at its base. Primary sedimentary structures, trace fossils, vertical successions of facies, and sediment body geometries suggest that these rhythmically bedded sediments were deposited in a delta-front/distributary-mouth-bar setting. The overall thickness of the Magoffin Member is highly variable. Within the study area, it ranges in thickness from less than 25 m to greater than 40 m. Evidence indicates that in areas where the Magoffin Member is thickest, it tends to be sandier and contain rhythmite intervals that are thicker and more complete than in areas where it is thin. The member displays several orders of centimeter- to decimeter-scale cycles that are consistent with semi-diurnal, diurnal, semi-monthly, and monthly tidal periodicities. Halfsynodic (semi-monthly neap-spring-neap) and anomalistic (monthly) lunar periodicities are manifested by the systematic thickening and thinning of the shorter duration cycles. The rhythmite interval ranges in thickness from less than 5 m up to 20 m and records up to four months of deposition. Accumulation rates for the rhythmites typically ranged from 20–100 cm per neap-spring-neap event, but reached rates of over 30 cm per day in areas where the Magoffin Member is thickest and the most proximal deltaic facies are preserved. The nature of tidal bundling suggests that these rhythmites accumulated in a mixed, dominantly semi-diurnal tidal system where both lunar phases and lunar declination influenced tidal cyclicity.