CYCLOTHEMIC OFFSHORE “CORE” SHALES AS RECORDS OF CLIMATE DYNAMICS AND INTERGLACIAL DURATIONS OF THE PERMO-CARBONIFEROUS ICE AGE
The offshore core shales of Midcontinent North American cyclothems provide a record of climate dynamics that potentially constrains the duration of interglacials of the Permo-Carboniferous Ice Age. Although thin (<1 m), these mm-laminated, organic-rich (>10 wt% TOC) shales may represent large portions of the total time of accumulation of complete, 3- to 10-m-thick cyclothems. The present study examines four offshore shales of early Late Pennsylvanian age, all exhibiting internal cm-scale cyclicity expressed as variations in bulk density, TOC, and trace-element concentrations. The Hushpuckney (52 cm thick; Swope cyclothem), Stark (58 cm; Dennis), Muncie Creek (42 cm; Iola), and Eudora shales (40 cm; Stanton) contain about 12, 12, 11, and 8 such cycles, respectively. Relevant observations include: (1) organization of cm-scale cycles into two or three sets per core shale, reflecting a hierarchical origin; and (2) thinning of individual cm-scale cycles toward the midpoint of each core shale and thickening above it, reflecting systematic, depth-related sedimentation rate changes. Paleoenvironmental analysis indicates that variations in terrestrial organic fluxes and pycnocline strength, both related to precipitation and runoff into the Midcontinent Seaway, controlled cycle genesis (Algeo et al., 2004, Chemical Geology, 206:259-288). The ubiquity and regularity of these cycles favor a quasiperiodic, allocyclic mechanism, probably Milankovitch orbital forcing. If the principal forcing was the ~20-ky precession cycle, as seems likely at low (~5-10°N) paleolatitudes, then individual core shales represent ~160 to 240 ky, or about half of the 413-ky long-eccentricity cycle inferred to be a major control on cyclothem formation (Heckel, 1986, Geology, 14:330-334). As core shales record the late transgressive to early regressive phases of coeval glacio-eustatic cycles, they were of longer duration than the interglacials they encompass. Interglacials, associated with maximum eustatic highstand and climatic warmth, may be recorded in core shales as TOC maxima (owing to sediment starvation) and shifts toward more aluminous siliciclastics (owing to enhanced chemical weathering). These features are confined to 6- to 12-cm-thick intervals, or about two cm-scale cycles, in the middle of core shales, implying a duration of ~40 ky for Late Pennsylvanian interglacials.