| Creator |
|
|
|
|
|
| Language |
|
| Publisher |
|
|
|
| Date |
|
| Source Title |
|
| Vol |
|
| Issue |
|
| First Page |
|
| Last Page |
|
| Publication Type |
|
| Access Rights |
|
| Crossref DOI |
|
| Abstract |
For the purpose of understanding depositional and tectonic settings of cherts around the Akiyoshi limestone Group (Visean to Guadalupian), Southwest Japan, this paper documents their distribution, rel...ationships to other rock units, stratigraphy, petrography, and age. The cherts occur as slabs and blocks of varied sizes and forms, and are complexly juxtaposed with and enclosed in pervasively sheared mudstones and sandstones. Geological and paleontological evidence suggests that the terrigenous clastic rocks are of Iate Guadalupian to probably post-Guadalupian age. The cherts and associated rocks exhibit the following stratigraphic sequence: (1) lower part (20 m thick) composed mainly of reddish volcaniclastic rocks with thin chert interbeds in the upper part. (2) upper part (up to 100 m) consisting of bedded or massive cherts. The cherts are characterized by a copious amount of siliceous sponge spicules and by admixture of volcanic detritus in varying degrees, particularly hl the lower part. The radiolarian dating indicates that the major part of the cherts ranges in age from Wolfcampian to Guadalupian. At several levels of the volcaniclastic rocks-chert sequences there occur limestone lenses with siliceous limestone envelopes which have resulted from mixing in the unconsolidated state of sponge spicule-rich sediments and calcitic skeletal debris exogenetically derived from the Akiyoshi carbonate buildup of shallow-water origin. Those of the lowest part yield fusulinaceans of Visean age, thereby the volcanoclastic-chert sequence is deduced to be coeval with the almost whole sequence of the Akiyoshi Limestone Group. All lines of evidence indicate that the cherts accumulated on the lower flank and foot of a volcanic seamount capped by a reef complex, the Akiyoshi Limestone Group, and on the surrounding ocean floor. These rocks formed in a realm far removed from in6ux of terrigenous detritus. Based on lithologic affiliation and age, rocks of the area are grouped into two major tectonostratigraphic units of completely different lithology; the one composed of rocks of oceanic setting including a greenstone-reef limestone complex and coeval deep-water sediments represented by the spicular cherts, both ranging in age from Visean to Guadalupian, and the other consisting of terrigenous clastic rocks of late to post-Guadalupian age. Complex juxtaposition of these two units is interpreted most likely as the result of collisional accretion of a seamount accompanied by a reef complex and spicular cherts toward terrigenous clastic rocks that accumulated within a trench. Accretionary process of these rocks probably took place during Late Permian to Early Triassic times.show more
|
Export Mendeley
26_01_P051