--> Tidal-Channel and Flood Tidal-Delta to Mud-Flat Lithofacies Associations and Sequence Stratigraphy of the Famennian Three Forks Formation, Williston Basin, North Dakota

AAPG ACE 2018

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Tidal-Channel and Flood Tidal-Delta to Mud-Flat Lithofacies Associations and Sequence Stratigraphy of the Famennian Three Forks Formation, Williston Basin, North Dakota

Abstract

Long-term sea-level fall during the late Devonian (Famennian) isolated much of the Williston Basin into a restricted intrashelf basin where dolomite, siliciclastic, and evaporite facies of the Three Forks Formation were deposited. Facies successions record abrupt basinward and landward shifts of restricted shelf, channeled-peritidal, and arid coastal mudflats during 3rd-order relative sea-level changes.

Our interpretation of environments hinged upon recognition of normal depositional processes and facies signatures associated with transgressive ravinement, tidal channel/point-bar migration, and tidal-delta progradation. Major lithofacies associations and environments are (1) massive to faintly laminated red mudstone with scattered mud-clasts and occasional fitted-breccia layers – supratidal mudflats with runoff channels, (2) nodular to laminated anhydrite in red, green, and gray mudstone (lower Three Forks) - salina and supratidal sabkha (3) crudely bedded/burrowed/disturbed rudstone-floatstone and wackestone - tidal-channel floor/lag with clasts derived from bank-margin collapse, (4) flaser-wavy-lenticular laminated, mud-cracked, tan dolosiltite-green shale - point-bar (inclined heterolithic strata), tidal channel levee and levee/pond margin, (5) laminated to wave/current rippled tan dolosiltite with Skolithos - tidal-delta mouth bar, mid-channel bar, and levee crest (6) green, silty dolomudstone - restricted lagoon and pond and (7) gray, silty dolomudstone - restricted lagoon.

We have identified six Three Forks sequences, spanning ~11 m.y. in the Famennian Stage. Famennian 5 and 6 sequences comprise the upper Three Forks reservoir. Lowstand deposits consist mainly of lithofacies associations 1, 6, and 7 representing restricted lagoon and supratidal environments. A sharp surface caps the lowstand deposits, and is followed above by transgressive tidal-channel deposits. These include one or more fining-upward bedsets of dolosiltite rudstone and/or floatstone of lithofacies association 3, passing upwards into either accretionary point-bar beds of lithofacies association 4, or tidal-delta mouth-bar and mid-channel bar dolosiltites of lithofacies association 5. A provisional maximum flooding surface is interpreted at a burrowed mudstone marking the turnaround to coarsening-upward bedsets of progradational tidal-delta levees, sandflats and mudflats (lithofacies association 4 and 5) above restricted lagoon and ponds (lithofacies association 6-7).