Microfabric Study of Fine-grained Sediments: Observations from the Amazon Subaqueous Delta

Abstract

ABSTRACT Sedimentary microfabric (millimeter-scale) is examined in thin sections of fine-grained sediments from the Amazon subaqueous delta. Coarse silt (4-6 ) laminae are abundant in the proximal portion of the Amazon subaqueous delta and are composed of numerous grain types, including quartz, feldspar, heavy minerals, wood fragments, and fecal pellets. In distal areas, the coarse silt fraction is absent (because of preferential accumulation of coarser material nearer to the source), and laminae generally are thin ( 0.1 mm) and are composed of fine quartz silt. Most silt laminae are observed within a matrix of platy minerals, and basal contacts are generally sharp, indicating scouring of th sea bed during formation of the laminae. Silt laminae form as a result of changes in sediment supplied from suspension and from particle-sorting processes operating near and on the seabed. Extinction phenomena of clay minerals and mica in thin sections reveal a unique category of sedimentary structures that is not observable using other (e.g., radiographic) techniques. Two extinction patterns (termed plasmic fabric) are observed: 1) a layered fabric that consists of horizontal layers of relatively coarse, platy minerals, which are interlayered (0.1 mm vertical scale) with unoriented, finer platy minerals, and 2) a uniform fabric with constituent minerals uniformly oriented parallel to bedding (on vertical scales of at least 1 mm). The fabric types represent primary (physical) sedimentary structures; however, the mechanisms for their formation are uncertain. The effect of bioturbation on microfabric is observed in thin sections where burrows disrupt silt laminae and erase evidence of the plasmic fabric. In the seaward portion of the Amazon subaqueous delta, the extent to which bioturbation alters physical structures depends on the ratio of biological mixing rate to sediment accumulation rate. Ratios greater than about ten indicate nearly complete homogenization of the sediment. The nature and distribution of microfabric in the Amazon subaqueous delta provide important information for interpretation of ancient mudrocks.