PALEOMAGNETISM AS A TOOL FOR DETERMINING DIAGENETIC, DEPOSITIONAL, AND EROSIONAL EVENTS AS RECORDED IN SEDIMENTARY AND IGNEOUS ROCKS

Abstract

Paleomagnetism is a useful means to study the depositional and diagenetic history of any formation. If a formation holds a geologically stable, ancient magnetization, the magnetization, and thus its origin, relates a specific event that the rock has undergone. Accurate dating of such events is difficult, particularly in the absence of stratigraphic controls, and re-creation of the history of a formation is crucial in understanding its importance in the rock record. Paleomagnetic information can be used to assess the timing of deposition of a unit or the timing of a diagenetic event that may have affected the formation of interest.

This study consists of three independent components that uses the timing of magnetization acquisition to characterize a specific event in the history of the formation of interest. The first study addresses clastic dikes of previously unknown age and origin that are exposed in the Pikes Peak Granite along nearly 175km of the Front Range in Colorado. These dikes are spatially associated with north- and northwest-striking thrust faults along the east margins of the range. Paleomagnetic demagnetization data from the dikes indicate the presence of a complex, multicomponent magnetization. A characteristic remanent magnetization (ChRM) with easterly declinations and moderate to steep inclinations resides in hematite and corresponds to a late Precambrian to early Cambrian pole position when compared to the apparent polar wander path of North America. This early magnetization was probably acquired at or soon after emplacement of the dikes and is therefore interpreted as a primary magnetization. A southeast and shallow magnetization, also residing in hematite, yields a grouping of poles of late Paleozoic age. This magnetization is interpreted as a chemical remanent remagnetization (CRM), possibly associated with fluid flow driven by Ancestral Rocky Mountain orogeny. The second study investigates the origin of a CRM in zebra dolomite of the Devonian Guilmette Formation in Nevada. Zebra dolomite is common in Paleozoic carbonate platform sequences of the Basin and Range, and is linked to migration of warm basinal brines. The zebra dolomite of the Guilmette Formation holds a magnetization carried by magnetite that is inferred to be late Triassic in age. The timing of this magnetization may be associated with fluids driven eastward during the Triassic Sonoma orogeny. The third study evaluates a widespread late Paleozoic magnetization associated with hematite formation in concert with albitization of Precambrian crystalline rocks. This Permo-Triassic aged magnetization has been reported at several localities in Europe and in North America, and is associated with hematite precipitation by weathering-related fluids. The paleoclimate at the end of the late Paleozoic represented a shift to greenhouse conditions and probable enhanced chemical weathering. Determining the extent of this remagnetization can help to identify the paleotopographic surface, understand the Permo-Triassic paleoclimate, as well as determine the widespread extent of this chemical weathering remagnetization event.