Flute Casts
Introduction
Flute casts are among the most prized paleocurrent indicators in sedimentary geology - not merely because they show the orientation of flow, but because they show which direction the current was travelling. The asymmetric bulbous-nose geometry is a built-in compass: one end points upcurrent and the other points downcurrent, with no ambiguity. Finding a surface covered in flute casts immediately reveals both the flow axis and its sense, and a swarm of flutes on a single turbidite bed is one of the most reliable pieces of directional evidence a field geologist can encounter.
Description
Flute casts are elongated welts or ridges that have a bulbous nose at one end that flares out in the other direction and merges gradually with the surface of the bed. They occur singly or in swarms in which all flutes are oriented in the same general direction. On a given sole, the flutes tend to be about the same size. Flute casts on different beds can range in width from a centimetre or two to 20 cm or more, in height (relief) from a few centimetres to 10 cm or more, and in length from a few centimetres to a metre or more. The plan-view shape varies from nearly streamlined, bilaterally symmetrical forms to more elongate and irregular forms, some of which are highly twisted. [1]
Formation Mechanism
Flute casts form by the filling of depressions scoured in cohesive sediment by current eddies created behind some obstacle, or by chance eddy scour. This type of current scour produces asymmetrical depressions in which the steepest and deepest part is oriented upstream. When such depressions are subsequently filled, the filling forms a positive-relief structure with the bulbous nose oriented upstream. [1]
The mechanism is straightforward once the eddy-scour geometry is understood. A current eddy rotates in the lee of an obstacle, scouring a spoon-shaped pit whose deepest, steepest end faces the current. Sand or coarser sediment fills this pit shortly after - often from the same current event. After lithification and uplift, what remains on the underside of the sandstone bed is the cast of that pit, with the deepest (bulbous) end pointing in the direction from which the current came.
Paleocurrent Significance
Flute casts are excellent paleocurrent indicators because they show the unique direction of current flow - they are unambiguous in a way that groove casts are not. [1]
This unambiguity is their main advantage over groove casts. A groove cast tells you the flow axis but leaves two possible directions; a flute cast removes that ambiguity entirely because the bulbous nose always points upstream. In practice, a geologist measuring paleocurrents from a turbidite sole will preferentially record flute cast orientations as the most reliable data points.
Occurrence
Flute casts are particularly prevalent on the soles of turbidite sequences, but they are also present in shallow marine and nonmarine environments. They have been reported on the soles of limestone beds as well as sandstone beds. [1]
Related Topics
Groove Casts
Groove casts are the casts of long, narrow scratches left on a muddy seafloor when tools - shells, wood fragments, pebbles - were dragged along the bottom by a current. They are simple, robust...
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...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs, Sam Jr.
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