Emplacement-related fabric and multiple sheets in the Maiden Creek sill, Henry Mountains, Utah, USA

Abstract

The Maiden Creek sill is a small intrusion (∼1 km 2 in map view) that is part of the Tertiary igneous complex of the Henry Mountains in southern Utah. Field relationships indicate that (1) this sill has a complex map view geometry, consisting of an elliptical main body from which radiate several finger-like lobes; (2) the intrusion is composed of two igneous sheets, presumably representing distinct magma pulses; and (3) the current exposure of the intrusion corresponds closely to the original intrusive geometry. These relationships allow us to examine in detail the relationships between fabric development, intrusive geometry, and emplacement processes in this intrusion. This sill was emplaced into the flat-lying sediments of the Colorado Plateau during a time of tectonic quiescence. Fabrics within the intrusion can therefore be interpreted in terms of igneous emplacement processes. Fabric data are derived from field measurements, image analysis, X-ray computed tomography, and anisotropy of magnetic susceptibility. In the center of each sheet, magmatic fabrics are subhorizontal and poorly defined. In the outermost 5–15 cm of each sheet, solid-state fabrics are developed sub-parallel to the contact. The two distinct intrusive sheets have similar composition, thickness, and areal extent. We infer that emplacement of the first sheet established the initial map view geometry of the intrusion and the second sheet closely followed the contact between that first sheet and the host rock. The example of the Maiden Creek sill demonstrates that sheeted emplacement can be an important process even in the early stages of construction of small intrusions, in addition to other larger intrusions studied elsewhere.