How are saucer-shaped sills emplaced? Constraints from the Golden Valley Sill, South Africa

Abstract

Magmatic sill intrusions develop saucer-like geometries in layered sedimentary basins intruded by large volumes of magma. Saucer-shaped sill emplacement models are based on the analysis of the intrusion geometry and their spatial relationships with potential feeders, not on magma flow pattern. The Karoo Basin of South Africa hosts hundreds of saucer-shaped sills. Among these, the Golden Valley Sill is well exposed and displays connections with adjacent and nested saucers. A combination of detailed field observations and anisotropy of magnetic susceptibility measurements (AMS) were used to identify strain markers that can be interpreted in terms of magma flow directions. A total of 113 localities (six specimens per site), mostly including pairs of opposite sill margins, were sampled for anisotropy of magnetic susceptibility analyses. The magnetic properties were determined by measuring hysteresis loops and K-T curves. The majority of the localities display well-defined magnetic foliations that consistently dip outward from the center of the Golden Valley Sill. This orientation of the magnetic foliation most likely represents inflation/deflation cycles of the intruding sill that interacts with nonstatic enclosing walls. In addition, magma channels and ropy flow structures were identified and display an imbrication of the magnetic foliation indicating an outward magma flow direction. The observed magma flow geometries derived from macroscopic flow indicators and the AMS data correlate well and constrain an emplacement model for the Golden Valley Sill Complex. Our emplacement model integrates our observations with dynamic emplacement processes characterized by inflation and deflation cycles. Copyright 2008 by the American Geophysical Union.