Abstract

Very poorly sorted, non-laminated thin layers of clayey to silty sand designated as "a"-division of turbidites occur among thinly laminated, fine-grained spill-over deposits on the natural levees of the relatively steep upper channel segment of the Northwest Atlantic Mid-Ocean Channel (NAMOC) of the Labrador Sea. These layers are non-graded and the transition to overlying ripple-cross laminated silt or parallel laminated terrigenous mud is abrupt both in terms of grain-size and sorting. The "a"-division is interpreted as a head spill-over deposit from channelized turbidity currents. Turbulence in the current head causes a wide range of available grain sizes to be thrown up high enough so that lateral overflow over the channel banks can carry them to the levees where they are deposited as massive, ungraded sand layers containing granule to clay-sized material. These head-spill overflows may be supercritical at times. Subcritical body spill from the dilute upper parts of the (same or a later) flow generally deposits thinly laminated (d-division) or homogeneous (e-division) muds which represent the bulk of the spill-over deposits. Locally, body spill deposits include ripple cross-laminated Fine sand and silt (c-division), and rarely a- and b- divisions. The common occurrence of the "a"-division on the upper channel levees (channel slope greater or equal 0.002) and the scarcity of such layers further downchannel (slope gradient less than 0.002) supports Komar's (1972) choice of a critical slope gradient of 0.002 for head spill-over to occur. Although no other modern examples of head spill deposits have been recognized similar, but considerably thicker, deposits are known from ancient levee sequences.

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