Magnetostratigraphy of the Karoo Basin P/Tr Boundary- John Geissman - 2021 MagIC Workshop (1/19/21)
Channel: Magnetics Information Consortium (MagIC)
Date: 2021-01-20

"Paleomagnetism of the Upper Beaufort Group (Late Permian to Early (?) Triassic) Karoo Basin, South Africa: An effort to "see through" the effects of Early Jurassic (ca. 184-183 Ma) Karoo LIP magmatism and establish a reliable magnetic polarity stratigraphy near and across (?) the Permian/Triassic Boundary " by John Geissman, University of Texas at Dallas and University of New Mexico.

Talk presented at the virtual 2021 MagIC Workshop (https://earthref.org/events/MAGIC/2021/) - Rock and Paleomagnetism through Time and Space. Jan 19th-21st, 2021.


Abstract:
The thermal history of Karoo Supergroup strata, deposited in the Karoo Basin from the Late Carboniferous to Early Jurassic, has long been of interest, including work over the past few decades on possible unconventional hydrocarbon resources in parts of the succession. The early thermal history of the basin is variable, as studies have demonstrated a general north to south/southwest increase in burial diagenesis during early stages of metamorphism attending Cape Fold Belt tectonism. This pattern is made complicated by both regional and local effects of widespread mafic magmatism associated with emplacement of the ca. 183 Ma Karoo Large Igneous Province (LIP). The response of Karoo Supergroup strata to such thermal effects bears on extracting meaningful magnetic polarity stratigraphic records from these rocks. An ongoing multifaceted effort has attempted to better understand the timing of inferred environmental changes in the Karoo Basin, from Late Permian to possibly Early Triassic (?) time, as recorded in Beaufort Group strata. We have obtained a robust paleomagnetic data set from selected localities that preserve strata inferred to have been deposited across the Permian/Triassic boundary in an attempt to construct a polarity record for these sections that may encompass events associated with the end-Permian extinction. Magnetic polarity records inherently require the preservation of early- acquired (primary) remanence (RM); yet demonstrating such preservation in Karoo strata is challenging. This is due, in part, to the fact that paleomagnetic field directions for the Early Jurassic (Karoo LIP) are similar to directions of Permian/Triassic age. In our work, we obtain multiple independent samples (i.e., 7 to 10+) per individual horizon (bed) to assess the uniformity of the RM. In most cases, independent samples are obtained by conventional field drilling methods.  However, numerous critical stratigraphic intervals are dominated by very friable hematitic siltstones and mudstones, and these have been sampled by carefully placing north- and up-oriented small fragments into ceramic cubes (20 mm on a side) and then cementing the fragments into place using non-magnetic alumina cement.  Strata at the Old Lootsberg Pass (OLP) and Tweefontein (TWFT) localities, Eastern Cape Province, are characterized by a north-northwest seeking, moderate to steep negative inclination RM, interpreted to be of normal polarity; typical initial RM intensities range from ~1 to 5 mA/m.  In most cases, this RM persists in progressive thermal demagnetization to about 580C, the maximum laboratory unblocking temperature (Tlub) for pure magnetite.  However, some beds uniformly show dual component behaviour, with complete unblocking of a normal polarity RM by about 425C and, thereupon, the isolation of a south-southeast, moderate to steep positive inclination (reverse polarity) RM.  We interpret this fact to indicate that, for these sections, any RM persisting above Tlub of about 425C is pre-Karoo LIP in origin, and likely primary. If the RM unblocked below about 425C is a thermoviscous RM then, based on theoretical relaxation time/magnetization blocking relations for pure magnetite, these rocks were heated to ~150-300C for ca. 1 Ma (+/-) (temperature estimates vary according to relaxation time/RM blocking relations).  The Beaufort Group section at the Bethulie (BT) locality (26 15’45”E, 30 25’0”S), Free State Province, is cut by several thin (less than 2 m wide) Karoo LIP dikes (red traces in sampling locality below), and permits numerous detailed contact tests. All of the mafic dikes in this area yield a characteristic RM that is of east-southeast declination and shallow positive inclination, suggesting that they were emplaced concurrently. Strata well-removed from obvious dikes show both normal and reverse polarity characteristic RM. If this overprint RM is of thermoviscous origin, our observation also implies that Bethulie strata experienced elevated temperatures (i.e., ~ 100 to 250C), if we assume a ca. 1 Ma (+/-) duration of elevated temperature.  Contact tests are positive, in that host strata adjacent (within less than 1-2 dike widths) to dikes have been thermally remagnetized and have enhanced initial RM intensities. Hence, documentation of a primary RM in Beaufort Group strata, which appears in some areas to be preserved, requires careful laboratory and field-based study.