ABSTRACT: Variations in the Lithologic Expression of Sequence Boundaries in an Upper Middle Carboniferous Cyclic Mixed Carbonate-Siliciclastic Succession, Midcontinent U.S.A.

FRENCH, JOHN A., Kansas Geological Survey, Lawrence, KS

Sequence boundaries related to subaerial exposure during glacial-eustatic lowstands are variably developed in three 2 to 30 m thick Lower Missourian depositional sequences in southeastern Kansas. Individual sequence boundaries are developed on either carbonates or siliciclastics, and their expression varies both regionally and locally. Because the identification of subaerial exposure surfaces is important to the recognition of depositional sequences, understanding of the characteristics and variability of such surfaces is important.

The basal carbonate-dominated Hertha depositional sequence is typically capped in shelfward locations by a zone of rhizocretions and/or vertical shale-filled fissures overlain locally by laminated crusts. An unfossiliferous blocky mudstone (paleosol?) may occur above this horizon. The overlying Swope sequence boundary caps either a shallowing-upward carbonate or a shale and typically is characterized by (1) a rubbly-weathering, rooted zone overlain by a blocky mudstone in relatively high-shelf positions; (2) a zone of oomoldic porosity in places containing microkarstic features, overlain locally by a blocky mudstone, at high and mid-shelf positions; and (3) blocky mudstone overlain by coal within a shale in low-shelf positions. In places this boundary is cryptic, revealed only by deta led petrography and stable isotope analyses. The overlying Mound Valley sequence is characteristically topped by a blocky mudstone/coal couplet that (1) merges with the Swope sequence boundary landward; (2) caps a shallowing-upward carbonate unit in mid-shelf positions; and (3) occurs within a sandy siliciclastic unit that overlies the carbonate basinward. This latter occurrence within a lithostratigraphic unit is important because this unit is a hydrocarbon reservoir.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)