[HCG27-P04] Features of parallel lamination in turbidite sandstones in the Upper Cretaceous Izumi Group in the southern part of the Awaji Island, southwest Japan
Keywords:turbidite, parallel lamination, formative condition
The Izumi Group was subdivided into three lithological parts; the northern marginal part, main part, and southern part. The main part of Izumi Group consists of alternating beds of sandstone, mudstone and conglomerate, and has been interpreted as submarine fan deposits. The turbidite sundstones was sampled from Anaga and Kita-ama formations of the main part of the Izumi Group. The intervals of laminae and oscillation of the grain-size distribution in parallel lamination of turbidite sandstones were measured by automated image analyses.
As a result, it was suggested that parallel lamination in turbidite sandstones can be classified into two types A and B. Type A lamination shows the abrupt change in grain-size at the boundary between laminae, while Type B shows gradual change. In naked eye observation, the former lamination is clearly recognized and the later lamina is unclear. The type B lamination occurs dominantly in turbidites observed in the study area.
Comparing with existing results of flume experiments, we interpret that two types of parallel lamination reflect difference in bed-aggradation rates. In general, the rate of bed-aggradation due to deposittion from turbidity current decreases with time. The fact that the occurrence of Type A lamination is limited in the upper interval of turbidites implies that there is a threshold value of bed-aggradation rate to from Type A parallel lamination. In future, to reconstruct quantitative paleo-flow conditions from characteristics of parallel lamination, we will examine the process and conditions of formation of parallel lamination by the model calculation with further analysis of actual turbidite sandstones.