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Radiolarian age and geochemical characteristics of the Permian bedded chert sequence in the Soi Dao area, Chanthaburi, Southeast of Thailand

Phromsuwan Waraphorn, Kamata Yoshihito, Ueno Katsumi, Charoentitirat Thasinee, Sardsud Apsorn

Geotourism / Geoturystyka

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2023

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nr 1-2 (72-73)

56--56

EN

Permian chert and siliceous mudstone in the Soi Dao, Chanthaburi Thailand are extracted the details on radiolarian assemblage and age, change of depositional environment, and geochemical characteristics. Permian radiolarians were obtained in three study areas (ASD01, ASD14 and ASD09); which radiolarian age of each section is as follows: ASD01: Early Asselian to Early Sakmarian, ASD14: Late Sakmarian to Artinskian, and ASD09: Capitanian to Early Changhsingian. Considering the lithofacies, ages, and chemical composition of the rocks, a preliminary stratigraphy consisting of basaltic rock, radiolarian bedded chert, siliceous mudstone, and coarse-grained clastic of alternation of sandstone and mudstone in ascending order can be reconstructed. Data on geochemistry analysis, particularly chondrite-normalized REEs patterns of chert and siliceous mudstone, present a gradual change in that degree of the Ce negative anomaly decrease toward the stratigraphical upper position. These changes indicate that the depositional site of the Permian rocks transferred from a state of high hydrothermal activity to a state of weakened activity and that the influx of terrestrial clastics increased. Permian bedded cherts accompanied by basalts and siliceous mudstones recognized in the study area closely resemble to the Paleo- ‑Tethys bedded cherts in terms of their lithofacies and microscopic features; however, their depositional period is much shorter than that of the Paleo-Tethys, indicating that it was deposited in another oceanic basin. The chemical compositions also show that the influence of hydrothermal activity weakened from the strong state, and the terrigenous clastics rapidly supplied.

2

Mid-oceanic seamount carbonates in Eastern Paleotethyan suture zones

Ueno Katsumi

Geotourism / Geoturystyka

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2023

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nr 1-2 (72-73)

75--76

EN

Mid-oceanic seamount-capping (atoll-type) carbonates make a popular stratigraphic entity in the geology of Japan since they are often seen as various-sized (but usually large and typically huge) exotic blocks within ancient (mostly Permian to early Cretaceous) accretionary complexes distributed in the Japanese Islands. These carbonates consist of very thick and pure (in the sense that it lacked input of continental detritus), usually massive and fossiliferous, shallow-marine limestone, and rest on oceanic-island basalts (OIB) of hot-spot origin, formed in the Panthalassa Ocean. Stratigraphically, they comprise a unique sedimentary succession that records long-term (sometimes over 80 myr.), continuous, shallow-marine environmental and biotic changes during late Paleozoic and early Mesozoic times of the oceanic sector with a stable tectonic setting, and can only be found within the accretionary orogen in the context of Ocean Plate Stratigraphy (OPS). Thus, the mid-oceanic seamount carbonate succession is a “surefire” geological item for the investigation of the ancient subduction zone and suture zone. On the basis of my research expertise working on these mid-oceanic carbonates in Japan over many years, especially in the Carboniferous–Permian Akiyoshi Limestone known as the most typical seamount-capping atoll-type carbonate body in the Panthalassa Ocean, I exported this, essentially “made-in-Japan” and “cultivated-in-Japan”, geological concept of “mid-oceanic seamount carbonates within the accretionary orogen” to Southeast Asian geology, for better understanding the general geotectonic subdivision and evolution of the relevant region, especially for clarifying the position of Paleotethyan suture zones and the geohistory of the Eastern Paleotethys Ocean. In today’s Southeast Asia, Paleotethyan mid-oceanic seamount carbonates are distributed in Northern Thailand and western Yunnan, SW China where Gondwana and Tethys meet together. Of these two regions, Northern Thailand is subdivided into three basic geotectonic domains; from east to west the Cathaysian Indochina Block, Sukhothai Zone (a Permian–Triassic island arc developed along the Indochina margin), and peri-Gondwanan Sibumasu Block. In the eastern part of Sibumasu, a geotectonically peculiar area called the Inthanon Zone can be identified on which Paleotethyan oceanic rocks including the Carboniferous–Permian Doi Chiang Dao Limestone of mid-oceanic seamount origin are widely distributed. This limestone succession, sometimes making kilometer-sized huge limestone blocks, is estimated to be 1000 m thick or more, and consists mostly of shallow-marine fossiliferous massive limestone without siliciclastic intercalation throughout. Basalts having intra-plate (oceanic volcanic island) geochemistry are observed at the base of the succession. Foraminifers, especially fusulines, are the fundamental fossil group for establishing its detailed chronostratigraphy, and they clarified that the limestone continuously accumulated from the Visean (middle Early Carboniferous) to the Changhsingian (latest Permian) over the time of 90 myr. In western Yunnan, the Changning–Menglian Belt is defined between the Lincang Massif (a Permian–Triassic island arc system formed along the easterly Simao Block with Cathaysian affinity) to the east and the peri-Gondwanan Baoshan Block to the west. The Changning–Menglian Belt, subdivided into the East, Central, and West zones, entirely has been regarded as a closed remnant (suture zone) of the Paleotethys Ocean, but actually it is only in the Central Zone where oceanic rocks are distributed. Paleotethyan mid-oceanic carbonates in this belt are called the Banka Limestone, which is over 1200 m in total thickness and generally massive and pure, being free from continental siliciclastic input for the entire succession spanning nearly 90 myr. Foraminiferal (mostly fusuline) biostratigraphy suggested continuous deposition ranging from the Visean to the Changhsingian without significant hiatus in the succession. Thus, the Banka Limestone in western Yunnan is exactly correlated in view of lithostratigraphy, chronostratigraphy, and tectonostratigraphy to the Doi Chiang Dao Limestone in Northern Thailand. In a broad geotectonic perspective, the Paleotethyan oceanic rocks including the Doi Chiang Dao Limestone, distributed in the Inthanon Zone are considered to form various-sized tectonic outliers upon autochthonous basement rocks of Sibumasu now, which consists of early Paleozoic– Triassic sedimentary, meta-sedimentary, and igneous intrusive rocks. Similarly, those distributed in the Central Zone of the Changning–Menglian Belt are structurally resting by almost flat-lying faults (thrusts) upon siliciclastic rocks of the West and/or East zones, which presumably represent passive-margin (continental slope) sediments of the westerly, Gondwanan Baoshan Block. These mid-oceanic rocks are interpreted to have been once incorporated within an accretionary prism formed by the subduction of the Paleotethyan oceanic lithosphere beneath the Permian–Triassic island arc system represented by the Lincang Massif–Sukhothai Zone. The resultant collision of the Cimmerian (peri-Gondwanan) Sibumasu–Baoshan Block to the Cathaysian Indochina–Simao Block, thus the closure of the Paleotethys Ocean in present-day Southeast Asia, at around Triassic–Jurassic boundary time emplaced rocks of the accretionary complexes (containing Paleotethyan oceanic rocks as exotic blocks) onto the marginal part of the Sibumasu–Baoshan Block as large thrust sheets (nappe).

3

Geological origin of the Permian bedded chert succession distributed in Central Plain of Thailand

Kamata Yoshihito, Phromsuwan Waraphorn, Ueno Katsumi, Charoentitirat Thasinee, Sardsud Apsorn

Geotourism / Geoturystyka

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2023

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nr 1-2 (72-73)

33

EN

The geotectonic divisions, which are distributed as narrow zones in the north-south direction, are clear in the northern Thailand, but not clearly recognized in the Central Thailand. The Chao Phraya Plain, which occupies the central part of Thailand, is broadly covered with Quaternary sediments. Therefore, the basement rocks of the Paleozoic and Mesozoic are scattered in the form of residual mounds, and due to the lack of stratigraphic and age-determination data, the geological origin of these rocks have been not sufficiently discussed. In this presentation, we will report the results of a study on the geological belongings of bedded chert sporadically exposed in the Chao Phraya Plain, Central Thailand by the microfossil age and geochemical characteristics. The study areas are Thung Saliam (TS) (50 km northwest of Sukhothai) in the northern part of the plain and the Nakorn Sawan–Uthai Thani (NS-UT) area in the central part of the plain. About 20 outcrops of chert were surveyed in both areas. The cherts are distributed in scattered, and most of the cherts are recrystallized and are considered to have undergone contact metamorphism. Chert of the TS is well-bedded with red and the direct contact relationship is unknown, but tuff and limestone (marble) are exposed around it. Chert of the NSUT is also well-bedded with red, black, gray and milky white in color. Altered slatey shale and sandstone accompany the chert and form monadnocks. These cherts might be categorized into typical pelagic cherts because they contain radiolarian tests and sponge spicules in a matrix consisting of very fine clay minerals and microcrystalline quartz, and do not contain coarse-grained terrigenous materials. Sashida & Nakornsuri (1999) reported the occurrence of Pseudoalbaillella simplex, Ruzencevispongus sp., and so on from the TS chert and they assigned their age to Wolfcampian. Whereas, Saesaengseerung et al. (2007) report radiolarian occurrence of the Ps. loemntaria Assemblage (Artinskian) and Follicucullus scholasticus Assemblage (Capitanian-Wuchiapingian) from the NS-UT chert. Whole-rock chemical analysis was performed on three sections their ages were determined by radiolarians. In the Chondrite-normalized REE pattern, TS chert indicates negative anomaly of Ce, and NS-UT chert shows relatively flat and profile of the downward-sloping in the LREE. The geological age of the study sections and their REE patterns are similar to those observed in the Sa Kaeo area in southeastern Thailand. The Permian bedded cherts exposed in TS and NS-UT areas are often accompanied by thin layers of fine-grained siliciclastics and tuff, which are weakly metamorphosed and foliated. Since the chert itself has a relatively thin thickness (several meters to 10–20 m) in each outcrop, and basically has a north-south strike. These cherts occurrence and lithofacies of both areas are similar and indicate that cherts of both areas are geologically comparable as pointed out by Ueno et al. (2012). Ueno et al. (2012) clarified that the central part of Thailand, where the geotectonic division was unclear, can be divided into three geotectonic units from west to east: Sibumasu Block, Sukothai Zone and Indochin Block. However, the origin of the Permian chert has not been clarified. Paleozoic and Mesozoic cherts distributed in Thailand remind us bedded cherts deposited in the Paleo-Tethys from the Devonian to the Triassic. However, no cherts other than the Permian have been reported in the central part of mainland Thailand. Instead of the Paleo-Tethys chert, the Sa Kaeo-Chanthaburi suture and the Nan-Uttaradit suture are well-known as geological units containing Permian chert in southeastern Thailand. These sutures have been understood as a closed remnant of the Permian to Triassic back-arc basin stretched between the Indochina Block and the Sukhothai Arc. The fact that the Permian cherts of the central plain are distributed in the eastern part of the Sukhothai Zone and near Indochina Block suggests that these Permian cherts comparable to the Permian chert of the Sa Kaeo-Chanthaburi Suture. In addition to the geological evidence, the geochemical features of the Permian chert represented by the REE pattern are similar to those of the Permian chert in the Sa Keo area. Line of evidence mentioned above suggests that the Permian bedded chert distributed the central Thailand (TS and NS-UT areas) have originated to the Permian chert of the Sa Kae-Chanthaburi or Nan-Uttaradit sutures which is a remnant of backarc basin.