Sandstone Dikes and Sills

Introduction

Sandstone dikes and sills are secondary sedimentary structures that record a dramatic loss of grain support. When sand becomes liquefied - losing all frictional contact between grains as pore-water pressure eliminates effective stress - it behaves temporarily as a fluid and can be injected into fractures in surrounding rock, much as magma intrudes into country rock. The analogy with igneous intrusions is apt: sandstone dikes cut across bedding; sandstone sills run parallel to it. Both are distinguished from normally deposited beds by their crosscutting geometry and their internal fabric.

Description

Sandstone dikes and sills are tabular bodies of massive sandstone that fill fractures in any type of host rock. They range in thickness from a few centimetres to more than 10 m. They lack internal structures except for oriented mica flakes and other elongated particles that are commonly aligned parallel to the dike walls. ^[1]

The alignment of elongated particles parallel to the dike walls records the direction of flow of the sand slurry as it was injected. This is a primary kinematic indicator: the particles were oriented by the velocity gradient of the moving suspension, just as mica flakes align in a flowing river, but in this case the movement was through a fracture rather than along a channel surface.

Dikes vs. Sills

Sandstone dikes form by forceful injection of liquefied sand into fractures in the surrounding rock, commonly in overlying rock, though injection downward has been documented in some cases. ^[1]

Sandstone sills are similar features formed by injection of liquefied sand parallel to bedding. Sills may be difficult or impossible to distinguish from normally deposited sandstone beds unless they can be traced laterally into sandstone dikes, or traced far enough to reveal a crosscutting relationship with other beds. ^[1]

This identification problem is practically important: a geologist mapping a sedimentary sequence needs to know whether a sandstone body is a primary bed or an injection sill. The distinction matters for reconstructing the original stratigraphy and for understanding the timing of deformation. A sill that cannot be traced to a dike remains ambiguous - only its lateral continuity, or its contact relationships with surrounding beds, can resolve the question.

Triggering Mechanisms

Suggested causes of sand liquefaction that drive dike and sill formation include earthquake shocks, as well as triggering effects related to slumps, slides, or rapid emplacement of sediment by mass flow. ^[1]

All of these triggers share a common mechanism: they impose a sudden increase in pore-water pressure or a rapid stress change that temporarily destroys grain-to-grain contacts within a water-saturated sand body. Once effective stress reaches zero, the sand loses all shear strength and flows under the pressure gradient. The structures that result - dikes cutting upward, sills spreading laterally - record the escape paths of the pressurised sand-water mixture.

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