DEVONIAN K-BENTONITES AND TUFFS: A RECORD OF CALEDONIAN MAGMATISM FROM THE APPALACHIAN FORELAND BASIN (EASTERN U.S.)

Altered airfall tephras (K-bentonites and tuffs) in foreland basin sediments provide another source of data on the timing and character of magmatic activity. This data is especially pertinent in the Devonian of the central to southern Appalachians, where there are significant gaps in the record of Devonian igneous activity. The widespread distribution of some K-bentonites and tuffs provide high resolution timelines to analyze foreland basin responses (e.g., loading-induced flexure, changes in sedimentation regime) to orogenic activity.

Approximately 100 K-bentonites and tuffs are known from the Devonian of the Appalachian foreland basin. These include clusters of several closely-spaced beds, and additional beds scattered through the succession. Some K-bentonites occur widely across the eastern U.S. (e.g., some beds of the Tioga and Belpre clusters and the Center Hill K-bentonite); some are restricted to portions of the Appalachian Basin (e.g., Tioga Middle Coarse Zone); others are only regionally to locally recognized. The highest concentration of K-bentonites and tuffs occurs in the Eifelian Stage (>40 beds). The known distribution of Devonian K-bentonites and tuffs through time, by stage, is as follows (K-bentonites dates from Tucker et al., 1998, and Roden et al. 1990; simplified stage boundary ages from Kaufmann 2006).

Lochkovian Stage (lower Lower Devonian, ca. 418-412 Ma): Bald Hill K-bentonites, up to 15 beds, 417.6 +/-1.0 Ma. Pragian Stage (middle Lower Devonian, 412-409 Ma): No K-bentonites reported. Emsian Stage (upper Lower Devonian, ca. 409-392 Ma): Sprout Brook K-bentonites, up to 15 beds, 408.3 +/-1.9 Ma. Eifelian Stage (lower Middle Devonian, ca. 392-388 Ma): Tioga Middle Coarse Zone, up to 32 beds, 391.4+/-1.8 Ma; Tioga A-G K-bentonites, up to 11 beds, 390 +/-0.5 Ma; and additional beds scattered throughout the Eifelian. Givetian Stage (upper Middle Devonian, ca. 388-384 Ma): Three K-bentonites reported; several undocumented clay layers scattered through the succession may be K-bentonites. Frasnian Stage (lower Upper Devonian, ca. 384-376 Ma): Belpre K-bentonites, up to 6 beds, 381.1 +/-1.3 Ma; Center Hill K-bentonite, close to 376 Ma; and a few additional beds. Famennian Stage (upper Upper Devonian, ca. 376-361 Ma): One K-bentonite reported, in low Famennian strata (close to 376 Ma).