CAN A CONTINUOUS WATER-FLOW GENERATE TURBIDITES - RESULTS FROM ANALOG EXPERIMENTS

The arenite-pelite or limestone-marl alternations of turbiditic sequences are today considered as resulting from quasi-instantaneous turbidite flows separated by long, relatively calm periods, i.e. background sedimentation, during which marls or pelites are deposited. This is the classical interpretation of the turbidity concept. Considering the frequent incoherency of the sequences as well as the presence of silts in the sometimes normally graded pelites we propose an alternative explanation for the origin of turbiditic deposits which emphasizes the continuous nature of turbidity flows rather than alternating mechanisms. We therefore carried out experiments involving the reproduction of turbidites in a tank. For the first time, the turbiditic alternation of arenitic and pelitic facies has been formed from a continuous unidirectional current rather than from the intermittent currents usually used in previous experiments. The arenite-pelite alternations were produced by introducing a given quantity of sediment into the continuous flow at various stages. The basin (i.e., the tank) occassionally undergoes turbiditic pulsations whereas the supplying current remains completely continuous. The turbidity of the water current increases rapidly after sediment addition and decreases very slowly until all the sediment is deposited. The procedure was repeated several times at lengthening intervals and with various quantities of sediment. Arenites and pelites were deposited in spite of the constancy of the current. This situation corresponds well to that in a river, in which the sediment load variations are controlled by variations in floods which subsequently enter the basin. These experiments therefore demonstrate that turbidites can form under fluviatile-type conditions. The facies discontinuities within turbidites need not reflect drastic systematic variations in turbid flows but could result from turbidity variations within a continuous turbid flow. Because rivers are the main carriers of sediments from the continents to the basins, they could be the cause of the turbidite formation. Such conditions could explain many sequential aberrations within turbidites, such as intrasequence erosion, pelitic laminae within ripple foresets, and the high proportion of silts in the pelites between the turbiditic sequences. These pelites should not be considered as "background" sedimentation, but as the product of a genuine turbid flow such as that which produced the coarse deposits. This model leads to significant modification of the sequence interpretation of turbiditic basins, which will be much more linked to drainage systems rather than to a particular geodynamic context.