Anatomy of the bi-gradational contourite sequence: Case study from the Gulf of Cadiz

The bi-gradational sequence (C1-C5) is the standard facies model for fine-grained, mixed mud-sand contourites. Drilling in the Gulf of Cadiz during IODP 339 recovered over 4.5 km of contourites with over 1600 distinct contourite sequences, having an average thickness of 3 m (range 0.5–7.5 m). This study documents the past 1.1 My of contourite succession at IODP Site U1389, in which there are a total of 299 full and partial sequences, with a variable thickness of 0.13–10.6 m (mean 2.65 m) and estimated duration of 0.4–32 ky (mean 8 ky).

Two complete bi-gradational sequences have been analysed in detail. Primary sedimentary structures are absent, apart from some bedding-parallel sharp contacts and abrupt omission surfaces. Bioturbation is pervasive throughout, and a distinctive pattern of ichnofacies change is observed through each sequence. Textural trends show reverse to normal bi-gradation through the sequence: mean size ranges from 7 μm to 55 μm, sorting from 1.8 to 2.9 phi, skewness from −0.3 to +0.6, and kurtosis from mesokurtic to very platykurtic. Compositional trends based on mineralogy, inorganic and organic geochemistry vary systematically with mean size. Of the biogenic fraction, the proportion of foraminifera increases with mean size and dominates the coarsest fraction (>150 μm). There is no discernable trend in planktonic/benthic ratio, and the benthic foraminifera are all characteristic of the upper bathyal zone. Between 30 and 60% of the tests are broken. Both terrestrial and marine sources of organic matter are present, with the former more abundant after 1 Ma and the latter dominant from 2 to 1 Ma.

Collectively, these facets of the contourite sequence validate the dual controls exerted by long-term variation in bottom-current velocity and episodic changes in sediment supply (both clastic and biogenic). Of these, bottom current velocity is the more important. Estimates of bottom current speeds, based on the standard sortable silt (SS) proxy, for the two sequences range from approximately 12–24 cm s⁻¹ (with fluctuations of 1–5 cm s⁻¹). However, we would support the case for a refined method that takes into account the very fine sand fraction moved by bottom currents. Where current speed is relatively high then non-deposition/erosion occurred and an omission surface results. Such omission surfaces probably account for between 20% and 35% of missing section in the two studied sequences. Where current speed was relatively low, thick featureless mud is deposited with a dominant hemipelagic sediment supply and slow alongslope drift. Such hybrid contourite-hemipelagite muds are believed to be widespread in the ocean.