--> Revisited: A Lamina-Scale Geochemical and Sedimentological Study of Sediments From the Peru Margin (Site 680, Odp Leg 112)

AAPG Hedberg Conference, The Evolution of Petroleum Systems Analysis

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Revisited: A Lamina-Scale Geochemical and Sedimentological Study of Sediments From the Peru Margin (Site 680, Odp Leg 112)


This work was first published 26 years ago in a conference proceedings (from SUMMERHAYES, C. P., PRELL, W. L. & EMEIS, K. C. (eds), 1992, Upwelling Systems: Evolution Since the Early Miocene. Geological Society Special Publication No 64, pp 131‐149). The following is the abstract from the proceedings. In the authors’ opinions, this work contributed to our understanding of processes in source rock formation on a very fine scale, and demonstrated that organic matter preservation is not as simple as either “productivity and (or versus) anoxia” or autodilution, as much of the literature has made it out to be. Yet this paper did not receive much attention, perhaps because of where it was published. This poster will provide an overview of the paper, and summarize its current significance in terms of understanding processes involved in organic matter preservation and source rock formation. Individual laminae (mm‐cm) were isolated from late Pliocene‐Holocene sediments cored at Site 680 during ODP Leg 112 (Peru upwelling region). At all depths, individual laminae alternate between (1) pale, high porosity, clay‐poor, diatom ‘framework’ oozes containing a high proportion of complete, well preserved diatom frustules and (2) dark, low porosity diatomaceous muds containing abundant diatom debris and foraminifer tests. In some instances, detrital‐rich laminae with low amounts of diatom debris were also observed. Hydrogen‐rich organic matter is finely disseminated throughout the sediments, perhaps occurring as coatings on detrital and biogenic grains. Both carbon isotopic and pyrolysis‐GCMS analyses suggest that the organic matter is derived mainly from a marine algal and/or bacterial input. Py‐GCMS further shows that, even in the sediments from 4.91 mbsf, the organic matter is significantly altered from the composition of living organisms. Only in two organic‐ lean Pliocene samples was there any sign of a significant input of terrestrial organic matter. Relative to laminae deposited during the Pleistocene‐Holocene, late Pliocene laminae are depleted in TOC. This may reflect variations in primary productivity. TOC contents also vary between adjacent diatomaceous mud laminae in Pleistocene‐Holocene sediments. In these laminae, TOC is inversely proportional to diatom abundance (which is believed to be related to productivity) but correlates positively with detrital content. Variations in the TOC content of closely spaced laminae is apparently not controlled by variations in productivity or preservation, but probably by variations in the TOC content of particulate matter reaching the sediment‐water interface. Some parameters (organic S/C; δ34 S) vary on a lamina scale — no larger scale trends are apparent. These variations are presumably the result of short‐term, local changes in diagenetic conditions and are hard to quantify. In these H2S‐rich sediments, differential uptake of sulphur into organic matter may be caused by variations in the availability of specific, reactive inorganic sulphur species, perhaps polysulphides. This study shows that conventional geochemical analysis of bulk, or of widely spaced samples can hide significant variations in sediment composition and inferred palaeoceanographical conditions. Although bulk samples may be used to determine the gross characteristics of a sedimentary sequence, fine‐scale sampling is needed to define the processes which determined those characteristics.