Flame Structures
Introduction
Flame structures are sedimentary deformation features that record a density inversion - a dense sand layer deposited on top of less dense, water-saturated mud. The mud, being unable to support the weight above it, is squeezed upward into the sand in irregular, upward-pointing tongues that in cross-section resemble flames. They are one of a family of loading structures, closely related to ball-and-pillow structures and load casts, that together signal rapid burial of a poorly consolidated, fluid-rich muddy substrate.
Description
Flame structures are wavy or flame-shaped tongues of mud that project upward into an overlying layer - which is commonly sandstone. The crests of some flames are bent over or overturned; in general, overturned crests tend to all point in the same direction, though this is not always the case. Flame structures are commonly associated with other structures caused by sediment loading. [1]
Mechanism
Flame structures are caused mainly by loading of water-saturated mud layers. These mud layers are less dense than the overlying sands and are consequently squeezed upward into the sand. The orientation of the overturned crests suggests that loading may be accompanied by some horizontal drag or movement between the mud and the sand bed above it. [1]
The density inversion is the key driver. Normally, denser material rests on top of less dense material - a stable gravitational arrangement. When sand is deposited rapidly on unconsolidated mud, the gravitational order is inverted: the denser sand sits on less dense, fluid-charged mud. The system resolves the instability by allowing the lighter mud to rise. Where a simple vertical rise occurs, the resulting flame stands upright; where horizontal flow or current drag accompanies loading, the crests are swept in one direction, producing the characteristic overturned geometry.
Association with Other Loading Structures
The consistent occurrence of flame structures alongside other loading features - load casts, ball-and-pillow structures - reflects the same underlying condition: rapid deposition of sand onto poorly consolidated, water-saturated mud. Flame structures are in a sense the complement of load casts: as the sand sinks into the mud (forming the load cast), the mud is displaced upward and sideways, forming the flames between the descending sand masses. This genetic relationship means that flame structures and load casts typically occur together on the same bedding surface.
Related Topics
Sandstone
Sandstones make up 20-25 percent of all sedimentary rocks. They are common in geologic systems of all ages and are distributed throughout the continents. They occur in beds ranging in thickness...
Load Casts
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...
Ball-and-Pillow Structures
Ball-and-pillow structures record one of the most dramatic soft-sediment deformation events: a sand or carbonate bed breaking apart and sinking into the underlying mud. The resulting hemispherical...
Sedimentary Structures
Sedimentary structures are large-scale features of sedimentary rocks - including parallel bedding, cross-bedding, ripples, and mudcracks - that form as a direct result of depositional or...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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