Early Jurassic pillow lavas and palynomorphs in the Karoo of eastern Botswana

Abstract

Volcanic rocks in conformable contact with fossiliferous sedimentary strata apparently present excellent possibilities for the calibration of the stratigraphical record by radiometric age determination. Unfortunately, many continental sedimentary sequences are poor in zone fossils. For example, correlation of the desert sediments in the upper Karoo Sequence of southern Africa relies on broad lithostratigraphical divisions¹ although vertebrate fossils can also be used² in some areas. However, precise dating of the onset of Karoo vulcanicity is crucial to the description of Gondwanaland. Here we note a highly exceptional area in eastern Botswana where the top of the largely aeolian ‘Cave Sandstone’ and the base of the overlying ‘Stormberg Lavas’ can be dated by the combined use of palaeontological and radiometric methods. Fluvio-lacustrine sediments within the lowest Stormberg Lavas are conformably overlain by basalts (in places, pillow lavas) with a probable K–Ar minimum age of ∼181 Myr. Early Jurassic palynomorphs (Classopollis intrareticulatus) suggest that the maximum age of these sediments is in the Upper Sinemurian, between ∼195 and 200 Myr.