Load Casts

Introduction

Load casts are deformation features, not erosion features. They are produced when sand sinks unevenly into the water-saturated, uncompacted mud beneath it - not by any prior scour of the mud surface. Despite appearing on the same soles as flute casts and groove casts, they carry no paleocurrent information. Recognising them correctly matters because mistaking a load cast for a flute cast would lead to false flow-direction interpretations.

Description

Load casts are described as swellings ranging from slight bulges, deep or shallow rounded sacks, knobby excrescences, or highly irregular protuberances. They commonly occur on the soles of sandstone beds that overlie mudstones or shales, and tend to cover the entire bedding surface. They range in diameter and relief from a few centimetres to a few tens of centimetres. ^[1]

Distinction from Flute Casts

Load casts may superficially resemble flute casts, but they can be distinguished from flutes by their greater irregularity of shape and their lack of definite upcurrent and downcurrent ends. Load casts also do not display a preferred orientation with respect to current direction. ^[1]

These three distinguishing criteria - irregular shape, no defined ends, and no consistent orientation - collectively remove any ambiguity. A flute cast has a nose and a tail and points unambiguously upstream. A load cast has neither; it is simply a rounded protrusion occupying an area of the bedding surface, surrounded by similar protrusions covering the whole sole.

Formation Mechanism

Despite the name, load casts are not true casts - they are not fillings of a pre-existing cavity or mould. They form by deformation of uncompacted, hydroplastic mud beds owing to unequal loading by the overlying sand. Uncompacted muds with excess fluid pore pressures - or muds liquefied by an externally generated shock - can be deformed by the weight of sand that sinks unequally into the incompetent mud. Unequal loading forces protrusions of sand downward into the mud, creating the positive-relief features visible on the base of the sandstone beds. ^[1]

Load casts are closely related genetically to ball-and-pillow structures and flame structures - all three involve the same instability of dense sand over less dense, fluid-charged mud. Flute and groove casts may also be modified by loading, which tends to exaggerate their relief and destroy their original shapes, a complication to be aware of when interpreting sole surfaces. ^[1]

Occurrence

Load casts can form in any environment where water-saturated muds are quickly buried by sand before dewatering has time to occur. They do not indicate any particular environment, though they tend to be most common in turbidite sequences. Their presence on some bed bases and absence on others reflects the hydroplastic state of the underlying mud at the time of sand deposition - they will not form if the mud was already compacted or dewatered before the sand arrived. ^[1]

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