USING THE SEDIMENTARY RECORD FOR FORELAND THRUST SEQUENCING: EXAMPLE FROM THE SOUTHERN APPALACHIANS (Invited Presentation)

Identifying the sequence in which thrust structures formed historically relies on juxtaposition analysis and cross-cutting relationships, such as truncation of footwall structures by out-of-sequence thrust elements. Growth strata are instrumental for dating structure in synkinematic basin fills, and facies relationships also provide insight. Thermal maturity data help identify pre-, syn-, and post-kinematic deformation, and thus provide key evidence for structural evolution.

Application of these principles to the southern Appalachian thrust belt in the greater Black Warrior Basin reveals a distinctive thrust sequence and proves the age of the foreland thrust belt. Wiley Dome contains the frontal structure, and a major angular unconformity above a breakthrough fault-propagation fold dates the structure as Serpukhovian. The southern part of the Sequatchie Anticline is a detachment fold. Growth strata and facies distribution indicate that the structure was active during Bashkirian-Moscovian time, and structural relationships indicate that growth continued beyond deposition of the preserved rock.

The Birmingham Anticlinorium is a shale-cored fault-propagation fold; isomaturity lines are not folded, squarely cutting structure, indicating post-kinematic maturation and, hence, early deformation. Behind this structure, isomaturity lines in Pennsylvanian strata of the Cahaba Synclinorium are less strongly folded than the host strata, indicating synkinematic maturation and thus later deformation. Stratigraphic evidence indicates that Cahaba was part of a piggyback basin that became isolated near the Bashkirian-Moscovian boundary. The Coosa Synclinorium is separated from the Cahaba Synclinorium by the Helena thrust, which truncates numerous structures in the footwall. Coosa synorogenic stratigraphy lacks clear growth strata and has a pre-kinematic maturation pattern, indicating even later deformation.

Accordingly, each structure becomes younger toward the hinterland (Serpukhovian-Moscovian), and so the regional thrust pattern is out of sequence. This research helps demonstrate the power of the sedimentary record for unraveling thrust history, and similar approaches can be used for detailed thrust sequencing in other regions.