Parting Lineation

Introduction

Parting lineation is a primary bedding-plane structure that preserves a direct record of current direction. It forms quietly, without the dramatic deformation of convolute bedding or the turbulent character of cross-bedding, yet it is one of the most reliable paleocurrent indicators available in parallel-laminated sandstones. Its value is limited by an important ambiguity: it shows the axis of flow, but not which of the two opposing directions the current actually moved.

Description

Parting lineation, sometimes called current lineation, forms on the bedding surfaces of parallel-laminated sandstones. ^[1]

It consists of subparallel ridges and grooves a few millimetres wide and many centimetres long. The relief on the ridges and hollows is commonly on the order of the diameter of the sandstone grains. ^[1]

The grains in the sandstone typically have a mean orientation of their long axes parallel to the lineation. ^[1]

The grain-scale relief is what creates the visual lineation when beds are split along the lamination surface. Each ridge is a shallow train of grains whose long axes align with flow; the hollows between them are the gaps between those trains. The relief is so slight - barely one grain diameter - that parting lineation is often invisible on a weathered surface and only reveals itself on a freshly split bedding plane. This means that it is underreported in the rock record relative to its actual frequency.

Paleocurrent Significance

The lineation is oriented parallel to current flow, making it useful in paleocurrent studies in ancient sandstones. However, it shows only that the current flowed parallel to the lineation and does not indicate which of the two diametrically opposed directions was the actual flow direction. ^[1]

This 180° ambiguity distinguishes parting lineation from flute casts and groove casts, which are directional. A geologist using parting lineation alone can constrain a flow axis but must draw on other structures - or regional context - to resolve the direction. In practice, parting lineation is most useful when combined with asymmetric current ripples or foreset dip directions that remove the ambiguity.

Occurrence and Origin

Parting lineation occurs in newly deposited sands on beaches and in fluvial environments. In the ancient rock record it is most common in thin, evenly bedded sandstones. ^[1]

Its origin is clearly related to current flow and grain orientation, probably owing to flow over upper-flow-regime plane beds, but the exact mechanism by which parting lineation forms is not well understood. ^[1]

The association with upper-flow-regime plane beds is significant. Upper-flow-regime conditions exist when the Froude number approaches or exceeds 1 - fast, shallow, high-energy flow. This is the same flow regime that produces flat lamination rather than cross-bedding, which explains why parting lineation occurs on the surfaces of parallel-laminated sandstone rather than on the foresets of dunes or ripples. The exact grain-sorting and alignment process that produces the regular ridge-and-groove texture, however, remains an open question in sedimentology.

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