Dish-and-Pillar Structures

Introduction

Dish-and-pillar structures are dewatering features - they form as water escapes upward through rapidly deposited, water-saturated sediment during compaction. The dishes and pillars are genetically linked and almost always occur together. Both are direct evidence of rapid sedimentation, and they are most abundant in turbidites and other gravity-flow deposits where sediment arrives fast enough to trap large volumes of pore water.

Dish Structures

Dish structures are thin, dark-coloured, subhorizontal, flat to concave-upward, clayey laminations that occur principally in sandstone and siltstone units. The laminations are commonly only a few millimetres thick, but individual dishes may range from 1 cm to more than 50 cm wide. They typically occur in thick beds where they may be the only visible structures. They also occur in beds less than about 0.5 m thick, where they commonly cut across primary flat laminations and other laminations. ^[1]

Pillar Structures

Pillar structures generally occur in association with dish structures. Pillars are vertical to near-vertical, cross-cutting columns and sheets of structureless or swirled sand that cut through either massive or laminated sands - which also commonly contain dish structures and convolute laminations. They range in size from tubes a few millimetres in diameter to large structures greater than 1 m in diameter and several metres long. Pillars are not actually stratification structures, but are discussed alongside dish structures because of their close association and common formation mechanism. ^[1]

The Dewatering Mechanism

Dish and pillar structures both form by the escape of water during consolidation of sediment - they are dewatering structures, and both are the direct product of the same process operating in different spatial directions.

During gradual compaction, semipermeable laminae act as partial barriers to upward-moving water that carries fine sediment. The fine particles are slowed by the laminae and added to them, progressively thickening them and giving them their dark colouration - these are the dishes. Some water is forced to travel horizontally beneath the laminations until it finds an easier vertical escape route. That forceful upward escape produces the pillars. ^[1]

The concave-upward shape of the dishes reflects the geometry of water movement - the edges of a dish are where water is forced laterally, so fine-particle deposition is heaviest there, curving the lamina upward at its edges. The pillar connects to wherever horizontal water flow finally breaks through the lamina above - a structural weak point or a zone of higher permeability.

Occurrence

Dish and pillar structures were first observed in sediment gravity-flow deposits - turbidites and liquefied flows - and are most abundant in such deposits. They have since been reported in deltaic, alluvial, lacustrine, and shallow marine deposits, as well as from volcanic ash layers. ^[1]

Their presence in volcanic ash layers is notable: ash is deposited essentially instantaneously over large areas and retains enormous amounts of pore water. The dewatering process that follows is identical in character to what happens in a turbidite, and dish structures are common in thick ash-fall deposits for exactly this reason.

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