Tidal sedimentation in the mid-Pennsylvanian Breathitt Group, eastern Kentucky
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The Magoffm Member (Four Comers Formation, Breathitt Group) outcrops in eastern Kentucky as a coarsening-upward succession of rhythmically interstratified sandstone, siltstone, and mudstone. Primary sedimentary structures, trace fossils, vertical successions of facies, and sediment body geometries suggest that these sediments were deposited in a delta-front/distributary-mouth-bar setting. Within the study area, the Magoffin Member ranges in thickness from 20 to 40 m. Where thickest, the Magoffm Member tends to be sandier and contain rhythmite intervals that are thicker and more complete than where it is thin. The member displays several orders of cycles that are consistent with semi-diurnal, diurnal, semi-monthly, and monthly tidal periodicities. Half-synodic (semi-monthly) and anomalistic (monthly) lunar periodicities are manifested by the systematic thickening and thinning of shorter duration cycles. The rhythmite interval records up to 4 months of deposition. Accumulation rates for the rhythmites typically ranged from 1 to 7 cm per day, but reached rates of over 30 cm per day where the Magoffin Member is thickest and the most proximal deltaic facies are preserved. Tidal cyclicity was also studied within the Betsie Shale and Kendrick Shale Members of the Breathitt Group. The Betsie Shale Member displays semi-diurnal through monthly tidal cycles. The Kendrick Shale Member displays semi-diurnal through semi-monthly tidal cycles. The nature of tidal bundling within the Breathitt Group rhythmite successions suggests that they accumulated in mixed, predominantly semi-diurnal tidal systems where lunar phases and declination influenced tidal cyclicity.
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