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The AAPG/Datapages Combined Publications Database

Journal of Sedimentary Research (SEPM)

Abstract


Journal of Sedimentary Research
Vol. 74 (2004), No. 5. (September), Pages 620-630

Relationships Between Composition of Organic Matter, Depositional Environments, and Sea-Level Changes in Backarc Basins, Central Japan

Akiko Omura*, Koichi Hoyanagi

ABSTRACT

Sedimentary organic matter is a potential indicator of paleoenvironments. In this study we examine the relationship among composition of Previous HitkerogenNext Hit (insoluble organic matter), sedimentary environments, and sea-level changes in Miocene to Pleistocene sediments of the Niigata and Akita backarc basins, central Japan. Our primary analytical tool is a ternary diagram with apexes consisting of woody-coaly organic matter, herbaceous with pollen and spores, and amorphous organic matter (AOM) with alginite (organic-walled marine microfossils). The composition of Previous HitkerogenNext Hit plots into distinct regions on the ternary diagram: fluvial to estuarine, prodeltaic, shelf, slope and basin-floor submarine fans, and distal basin-floor sediments. The fluvial and estuarine sediments have high proportions of woody-coaly and herbaceous organic matter with pollen and spores, and a lesser proportion of AOM + alginite, because pollen and spores were mainly deposited in estuaries. Because abundant coarse-grained, terrigenous organic matter was supplied by delta distributaries, the prodeltaic sediments have high proportions of woody-coaly organic matter. The composition of Previous HitkerogenNext Hit in the shelf sediments is similar to the Previous HitkerogenNext Hit in slope and basin-floor submarine-fan sediments, as plotted on the ternary diagram. Both kinds of Previous HitkerogenNext Hit accumulations have high proportions of woody-coaly organic matter and AOM with alginite and lesser proportions of herbaceous organic matter with pollen and spores. This relationship suggests that turbidity currents supplied the terrigenous sediments. The sediments on the distal basin-floor contain high proportions of AOM.

Each pattern in the ternary diagram reflects a difference in hydrodynamic behavior, distance from land, and the supply of terrigenous organic matter. The sub-ternary diagram, which has apexes of WFA (weakly fluorescent amorphous organic matter), NFA (nonfluorescent amorphous organic matter) + FA (fluorescent amorphous organic matter), and alginite, further suggests the origin of AOM. The NFA in shelf sediments and WFA in distal basin-floor sediments are inferred to consist of terrestrial higher plant and marine organic matter, respectively. A delta.gif (54 bytes)13C value of Previous HitkerogenNext Hit rich in NFA (-24.6 and -27.3‰) suggests land plants, whereas Previous HitkerogenNext Hit rich in WFA (-20.0 to -23.6‰) suggests marine plankton. These inferences agree with those derived from the sub-ternary diagram. Furthermore, compositional changes of the Previous HitkerogenNext Hit in turbidites reflect relative sea-level changes, as seen by shifts in compositions on the ternary diagram. The use of ternary diagrams like those used here is recommended for future studies of Previous HitkerogenTop, depositional environment, and sea-level relationship.


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