ABSTRACT: Paleokarsting and Dolomitization Beneath the Sub-Kaskaskia (Early Devonian) Sequence-Bounding Unconformity, Northwest Illinois Basin Margin

John M. Kruger

The sub-Kaskaskia (Early Devonian) sequence-bounding unconformity is well exposed in outcrops in west-central Illinois and demonstrates the interplay between sequence-bounding unconformities, diagenesis, and reservoir potential. On the northwest Illinois basin margin, the sub-Kaskaskia unconformity separates middle Silurian shelf-facies carbonate rocks from overlying Devonian limestone, sandstone, and shale. Paleokarst and dolomitization related to pre-Kaskaskia exposure increased porosity and permeability in Silurian rocks well below the unconformity surface.

Pre-Kaskaskia sequence karst features include meter-thick breccias and sand-filled fissures that extend to 6 m beneath the unconformity. Breccias contain clasts of dolomite, chert, and limestone in a quartz sand matrix. Cross-cutting relationships indicate that dolomitization preceded karsting.

Massive pre-Kaskaskia sequence dolomite is best developed several meters below the unconformity. The dolomite forms distinct, massive, 2 m thick layers separated by undolomitized limestone. Dolomite layers can be traced for more than 40 km and commonly underlie extensive dissolution zones.

Where dolomitization and paleokarst overlap, they locally form meter-wide sand-filled cavities with excellent reservoir potential. These diagenetic porosity zones are best developed several meters below the unconformity surface. Outcrop exposures of pre-Kaskaskia karsting and dolomitization extend over an area of more than 500 km² and correlate with Devonian karst in the Marine Pool field and elsewhere on the pre-Kaskaskia carbonate platform.

The relationship between paleokarst and dolomite below the sub-Kaskaskia unconformity demonstrates that porosity developed during exposure events may be best developed well below unconformity surfaces.

AAPG Search and Discovery Article #90998 AAPG Eastern Section Meeting, London, Ontario, Canada, September 10-12, 1990