Origin of Convoluted Laminae

Abstract

Comparison of streaked-out " ripples " formed by the drag effects of a current passing over a watery, yet cohesive sediment bottom with convoluted laminae formed within beds of fine-grained sandstone suggests a common origin. Application of Bagnold's recent discoveries on the behaviour of cohesionless sediment in flowing fluids to the problem results in a modification of Kuenen's hypothesis of origin of the convolutions. According to the new interpretation, convolutions arise when formerly cohesionless sand grains become cohesive after deposition and respond to increased shearing due to higher current velocity by a decollementtype of adjustment with the plane or planes of adjustment located within, or in some cases at or below the base of the growing sandstone bed. Convolute " anticlines " in cohesive sand are thought to serve the same function as that played by current ripple-marks in cohesionless sand, i.e., they create additional bottom relief in order to increase the drag and restore equilibrium to the added shearing stress imposed by the current that cannot be counterbalanced by grain-tograin encounters.

The Sedimentology Of The Waitemata Group In The Takapuna Section,Auckland

“…The convoluted lamination in the Waitemata turbidites is simpler in style than much of that described from other formations, and the theory of formation proposed by Sanders (1960) fits the observed facts very closely. Briefly, Sanders's theory is based on experimental work by Bagnold (1956); he regards "anticlinal" convolutions as transverse ripples, formed in sediment whose grain size is transitional between coarse, cohesionless sand, and fine, cohesive silt and mud.…”

“…Convoluted lamination is a widespread feature of turbidites, various authors having described it in detail and discussed its origin (Kuenen, 1953b;Ten Haaf, 1956;Holland, 1959;Williams, 1960;Sanders, 1960). The convoluted lamination in the Waitemata turbidites is simpler in style than much of that described from other formations, and the theory of formation proposed by Sanders (1960) fits the observed facts very closely.…”

The Provenance and Emplacement of Upper Arenigian Turbidites in Co. Mayo, Eire.

“…5C). Convoluted lamination, observed in the lateral turbidites, never shows the anticlinal erosion and synclinal filling thatSanders (1960) describes but B C TEXT-FIG. 5.-Internal features of the lateral turbidites.…”

“…On the leeward side of streaked out ripples and flow casts overturned ripple drifted silts are commonly found (a similar relationship to that described by Lamont, 1957). Sanders (1960) has ascribed the formation of these structures to the shear effect produced by the passage of a density current across a layer of cohesive mud and as Sanders states (p. 412) " Proof that ripple-like forms in lutite were made during the passage of the current consists of evidence that bottom relief existed in the mud at the time sand was deposited." As in the cases described by Lamont and reiterated by Sanders, deposition of ripple drifted sediment on the lee sides of flow casts and allied structures demonstrates that such structures are syndepositional and not due to later mass sliding of a turbidite sheet on a slope.…”

Sedimentary structures and palaeocurrent directions from the Silurian rocks of Kirkcudbright

“…This may be due to a process of rucking of the cohesive fine-grained layer in response to current drag (see Sanders 1960) or to rhythmic transverse scouring as suggested by Djulynski and Sanders in their privately circulated paper. (b) Production of longitudinal ridges on the rippled surface during later and slower stages of flow when even the slight ripple slopes influenced the local direction of move ment and the residual current was in some cases channelled along the troughs.…”