Distinctive Properties of Turbiditic and Hemipelagic Mud Layers in the Algéro-Balearic Basin, Western Mediterranean Sea

Abstract

Two types of mud layers alternate in dominantly muddy cores of the southern Balearic Basin. Type A muds (a few cm to over 50 cm thick), macroscopically homogeneous, occur above turbidite sands or silt laminae. Type B muds (imperceptible to about 50 cm thick), comparatively coarse due to interspersed microskeletons, occur below turbidite sands or silt laminae, and lie above type A muds. The two types are distinguished in X-radiographs on the basis of texture and sedimentary structures. Type A and B mud layers in six cores were sampled at 1 to 8 cm intervals.

Type A muds are distribution graded (upward shift of the entire size distribution to finer sizes), continuing the upward grading of the underlying sand turbidites. A granulometric change occurs at the boundary with type B muds which contain sand (to 16 percent), largely tests of forams and pteropod shells. The sand fraction of type A muds (d" 1 percent) differs from that of type B in the proportion of terrigenous constituents and in remains of pelagic forams and of pteropods. Type B muds are not graded; their grain-size distribution is uniform. They have a higher (26 to 46 percent) carbonate content than type A (16 to 39 percent). In some instances, peak-height ratios of clay minerals change across the boundary between type A and type B mud layers. It is concluded that type A muds are turbiditic (deposited instantaneously), while B muds are hemipelagic deposits.

Carbon-14 ages were determined on the carbonate sand fraction of type B layers. The ages were plotted against the total sediment thickness above the dated samples in each core. A statistically significant correlation exists. However, when the turbiditic sand and mud layers are omitted and the ages are plotted only against the combined thicknesses of the hemipelagic type B layers, an even stronger correlation is obtained. The hemipelagic rate of sedimentation during the past 16,000 years approximates 10 cm/1000 years. The frequency of turbidity current incursions at a particular core location averages 3 per 2000 years.