Sedimentary Structures in Igneous Intrusions with Particular Reference to the Duke Island Ultramafic Complex
T N. Irvine, 1965. "Sedimentary Structures in Igneous Intrusions with Particular Reference to the Duke Island Ultramafic Complex", Primary Sedimentary Structures and Their Hydrodynamic Interpretation, Gerard V. Middleton
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Many igneous intrusions show layering formed by gravitational accumulation of crystals that is remarkably similar, both in variety and detail, to bedding in sedimentary rocks. Such layering occurs in most compositional types of intrusions but is especially common in mafic and ultramafic bodies. The examples considered specifically are from the Duke Island ultramafic complex in the Alaskan panhandle; the rocks are composed of olivine and clinopyroxene but, where layered, resemble graded–bedded turbidity current deposits.
The Duke Island complex comprises two major intrusions, and the layering which has been tilted and folded, has two ages—thus, layered blocks and fragments of olivine clinopyroxenite are included in stratified peridotite. The stratification is developed intermittently through a total section of about 2 miles thickness. Individual layers have been traced for 300 feet, and one continuously layered section is 1,500 feet thick and extends for 1,000 feet. Typical layers are 2 inches to 2 feet thick and are graded from grain sizes of 2–10 mm at their base to 0.1–1 mm at their top. The base of a graded layer is sharp; the top may coincide with the base of the next, but commonly passes into an intervening zone of thin laminae. Some layers of younger age resemble graded beds of slide conglomerate, their lower parts being largely olivine clinopyroxenite fragments. Other layering features are: loading and impact structures; scour–and–fill; local angular unconformities; streamlining over irregularities; slump structures; lateral grading; grain–size sorting similar to that in turbidite bedding; “sandstone” dikes; and “diagenetic” recrystallization.
The layering undoubtedly was formed by sedimentation from magmatic currents during extremely unstable conditions. The described phenomena illustrate that features of water–laid sediments can form in a greatly different environment in terms of the densities of the particles and transporting liquid, and the viscosity of the liquid.