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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