GSA Annual Meeting in Denver, Colorado, USA - 2016

Paper No. 261-1
Presentation Time: 9:00 AM-6:30 PM


GEISSMAN, John W., Department of Geosciences, University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080, GASTALDO, Robert A., Department of Geology, Colby College, 5807 Mayflower Hill Drive, Waterville, ME 04901 and NEVELING, Johann, Council for Geosciences, Private Bag x112, Pretoria, 0001, South Africa,

The thermal history of Karoo Supergroup strata, deposited from the Late Carboniferous to Early Jurassic, has long been of interest. Studies demonstrate a general N to S/SW increase in early burial diagenesis attending Cape Fold Belt tectonism, yet the pattern is complicated by widespread ca. 183 Ma Karoo mafic magmatism. A multifaceted effort to better understand the timing of inferred environmental changes in Beaufort Group strata from Late Permian to Early Triassic time includes obtaining robust paleomagnetic data from selected localities to construct polarity records, which inherently require the preservation of early- acquired (primary) remanence (RM). Proving such preservation in Karoo strata is challenging; field directions during Karoo magmatism are similar to those of Late Permian age. Multiple samples (7 to 10+) per horizon allow RM uniformity assessment. Strata at the Old Lootsberg Pass (OLP) and Tweefontein localities, Eastern Cape, yield N/NW seeking, moderate to steep negative inclination RMs (normal polarity) with RM intensities of ~1 to 5 mA/m. This RM usually persists in thermal demagnetization to ~580o C, the maximum laboratory unblocking temperature (Tlub) for magnetite. Some beds show dual components, with unblocking of a normal RM to 425-450oC and the isolation of a S/SE, moderate to steep positive inclination (reverse) RM. For the OLP section, any RM persisting above Tlub of about 425oC is likely pre-Karoo magmatism (primary,?). If the first unblocked RM is thermoviscous, theoretical relaxation time/magnetization blocking relations imply the rocks were heated to ~150-300o C (estimated thermal duration of ca. 1 Ma). Beaufort strata at Bethulie (BT), Free State, are cut by several Karoo dikes. Strata removed from dikes show both normal and reverse RM. Site BT15 yields a well-defined reverse RM with a weaker, superimposed normal RM fully unblocked below 400oC. If this overprint RM is thermoviscous, this implies that Bethulie strata experienced elevated temperatures (i.e., ~ 100 to 250oC+) for ca. 1 Ma (+/-). Contact tests show that strata adjacent (< 1-2 dike widths) to dikes are remagnetized with enhanced RM intensities (> 50 mA/m). Documentation of a primary RM in Beaufort strata, which appears in some areas to be preserved, requires careful laboratory- and field-based assessment.