Stratigraphic Heterogeneity of Lower Carboniferous Grainstones, Missouri Gene Rankey

Purposes Document facies and sequence stratigraphy of Carboniferous grainstone succession Illustrate internal complexity and heterogeneity within grainstones Provide framework for fracture and diagenetic study Rankey Mississippian, CSL Annual Review

Major Findings Succession forms an overall progradational highstand systems tract, with facies ranging from sub-wavebase to peritidal Grainstones have diverse geometries, but heterogeneity occurs in predictable paleogeographic and sequence stratigraphic settings Internal truncation and heterogeneity – create complex compartmentalization Rankey Mississippian, CSL Annual Review

Location Landward (N) –basinward (S) transect Flank of Illinois Basin Low-angle ramp Rankey Mississippian, CSL Annual Review

Stratigraphic Setting Rankey Mississippian, CSL Annual Review

Stratigraphic Setting Flooding surfaces ‘Parasequence Sets’ Shallow-up: progradational facies tracts

Facies Rankey Mississippian, CSL Annual Review

Indiana Analogs (courtesy Brian Keith, Indiana Geol. Survey) Facies Indiana Analogs (courtesy Brian Keith, Indiana Geol. Survey) Rankey Mississippian, CSL Annual Review

Facies Model Rankey Mississippian, CSL Annual Review

Parasequences Lower HST/ Downdip Upper HST Rankey Mississippian, CSL Annual Review

Grainstone Heterogeneity Several Scales: Laminae Sedimentary structures Beds Parasequences Rankey Mississippian, CSL Annual Review

Grainstone Heterogeneity Upper HST Updip areas Rankey Mississippian, CSL Annual Review

Grainstone Heterogeneity

Grainstone Heterogeneity Truncation common in upper HST

Grainstone Heterogeneity

Grainstone Heterogeneity Complex Tabular Rankey Mississippian, CSL Annual Review

Tabular Grainstone Facies: crinoid-bryozoan-skeletal grainstone Height: up to 1m Continuity: many continuous across 300+ m outcrops Geometry: planar base and top Sequence Stratigraphic Occurrence: lower HST Geographic Occurrence: more common downdip Rankey Mississippian, CSL Annual Review

Channeled Grainstone Facies: foram-rich skeletal grainstone; common sigmoids and muddy toes or drapes; thin mudstone layers/lenses Height: erode up to ~1.5 m of section Continuity: some greater than 50m across Geometry: concave base, planar top; multiple channels Sequence Stratigraphic Occurrence: most common in middle-upper HST of composite sequence Geographic Occurrence: more common updip Rankey Mississippian, CSL Annual Review

Sand Waves Facies: crinoid-foram grainstone; large-scale (up to 2 m) troughs Height: up to 3m Continuity: some greater than 60+ m across Geometry: convex top, planar base Sequence Stratigraphic Occurrence: most common just above flooding surfaces of high-frequency sequences, just below grey shale Geographic Occurrence: ubiquitous

Summary and Implications Succession forms an overall progradational highstand sequence set, with facies ranging from sub-wavebase to peritidal Internal truncation and heterogeneity – create complex compartmentalization Grainstones have diverse geometries, but heterogeneity occurs in predictable paleogeographic and sequence stratigraphic settings Rankey Mississippian, CSL Annual Review