Contourite cyclicity and deposition

Abstract

Cyclic depositional features are commonly developed in deepwater sedimentary facies. Stacked sequences in varied forms is the most obvious characteristic, which is related to complex variation in depositional conditions. This study introduces several geostatistical approaches to analyse the cyclical bi-gradational sequences of contourite deposits from IODP Expedition 339 in the Contourite Depositional System (CDS) in the Gulf of Cadiz. It analysed both the vertical sequences and their cyclicity and their lateral correlation, both qualitatively and quantitatively. Additionally, similar geostatistical approaches were applied to colour sequences in turbidite and hemipelagite deposits from the Benguela Current Upwelling System, SW African continental margin. This allowed comparison of sequences and cyclicity between the different deepwater facies types. It also demonstrated the general applicability of this method to deepwater sedimentary facies. This study systematically examined contourite bi-gradational sequences deposited at IODP Sites U1386 and U1387 between mid-Pleistocene to Recent. Transition probability analysis based on the lithological logs confirmed the statistical validity of typical bi-gradational sequences composed of coarsening-upward to fining-upward contourite divisions. These typically ranged in thickness from 0.5-5m. Variations caused by minor erosion and non-deposition resulted in incomplete sequences and complex sequences with multiple gradational divisions. The cyclic patterns of contourite bi-gradational sequences varied to some extent between each studied hole. The 3-layer-sequence (C1-C2-C4-C5) dominates in this study area, and a few sequences with more than 4 divisions include the C3 (sandy) division. Autocorrelation of sequence duration in all studied holes indicates the existence of long-term cyclicity of around 350 ky. The succession can be subdivided into 4 stages (0-350 ka, 350-700 ka, 750-1000 ka and >1000 ka) with alternating occurrence of more frequent and coarser sequences and less frequent finer-grained sequences. Cross-correlation of this long-term sequence frequency as well as of individual sequences shows moderately good but not perfect correlation between holes and sites. This study suggests contourite deposition at the study sites is controlled by both bottom current strength (speed) and sediment supply, both of which can most probably be related to a complex variation in paleoclimate evolution and orbital cycles In hemipelagite and turbidite-hemipelagite hybrid deposits beneath the Benguela Upwelling System off SW Africa, two colour sequence models (Hemipelagite Dominant Sequence and Turbidite Hemipelagite Hybrid Sequence) were developed based on light-dark variation related to organic matter content. Autocorrelation of sequence duration pointed out long-term cyclicity through the past 4 My (0-1 Ma, 1-2.5 Ma, 2.5- 4 Ma and >4 Ma), which can be correlated to stages in tthe long-term evolution of the Benguela Current Upwelling System. Lateral correlation between studied sites indicated differences of sequence frequency in time and space, which further contributed to reconstruction of the upwelling system. The geostatistical study of cyclicity in contourite bi-gradational sequences and its comparison cyclic characteristics of other deepwater facies (turbidites and hemipelagites), demonstrates that such geostatistical approaches can be an important technique to evaluate the basic sedimentary character of different systems and their numerical expression. It allows for comparison between facies types and for better correlation with other time series records, such as orbital climate patterns. Cyclic signatures can be correlated between different sites and help better understand the sedimentary processes involved in their deposition