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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 48 (1964)

Issue: 4. (April)

First Page: 533

Last Page: 533

Title: Sedimentary Structures in Layered Ultrabasic Rocks: ABSTRACT

Author(s): T. N. Irvine

Article Type: Meeting abstract


Many igneous intrusions show layering formed by gravitational accumulation of crystals that is, both in variety and detail, remarkably similar to the bedding of sedimentary rocks. Such layering occurs in most compositional types of intrusions but especially 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, look much like graded bedded turbidity current deposits. The intrusion is composite, and the layering, which has been tilted and folded, has two ages--thus, layered blocks and fragments of olivine pyroxenite are included in stratified peridotite. Over-all, stratification is developed intermittently through an or ginal vertical thickness of 2 miles. Individual layers have been traced for 300 feet, and one continuously layered section is 1,500 feet thick and extends 1,000 feet.

A typical layer is 2 inches to 2 feet thick and is graded from grain sizes of 2-10 mm. at its base to 0.1-1 mm. at its top; some mineralogical sorting results because the olivine crystals are generally finer and thus concentrated upward. The base of a layer is sharp; the top may terminate against the next stratum or may pass into an intervening zone of thin laminae. Some of the layers of younger age resemble graded beds of slide conglomerate, their lower part being largely olivine pyroxenite fragments.

Other layering features include: cross-layering, slump and deformation concurrent with accumulation, streamlining of layers over irregularities, load casts, general correlation between layer thickness and particle size, lateral grading, and "diagenetic" recrystallization.

The layering has undoubtedly formed because of magmatic currents during extremely unstable conditions. The currents were probably a convective type of overturn marked by the descent of dense, crystal-laden magma from the cooling roof and wells of the pluton, and it is likely that they were initiated and perpetuated by repeated slumping. The described phenomena illustrate that features of water-laid sediments can form in a vastly different environment in terms of the specific gravities of particles and transporting liquid, and the viscosity of the liquid.

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Copyright 1997 American Association of Petroleum Geologists