Depositional and Tectonic Evolution of Lampang Group in Lampang and Phrae Provinces, Thailand

Abstract

The sedimentary rocks, which are distributed in the Lampang and Phrae Provinces, central of northern Thailand, are Triassic marine sedimentary rocks of Lampang Group. These rocks distributed in an elongate shape in the northeast-southwest direction in two main sub-basins, Lampang Sub-basin and Phrae Sub-basin of the Sukhothai Arc. This study focused on the link between depositional history, paleoenvironment, and tectonic settings of the Lampang Sub-basin. In addition, the provenance, distribution, transitional process of sediments, and fossils were studied in order to analyze and reconstruct the depositional evolution model of the Lampang Group. The Lampang Group in Lampang Sub-basin consists of five formations, Phra That, Pha Kan, Hong Hoi, Doi Long, and Pha Daeng Formations from base to top. The method of this study is divided into two main parts, including field observation and laboratory study. The stratigraphic study, sedimentary facies, paleocurrent observation, and geological structures are based on thirty study sections in the Lampang and Phrae Provinces. The stratigraphic study, bed thickness measurement, distribution, and stratified sampling method, in which samples are collected where there is a change in lithological characteristics, are employed here. In the laboratory studies, sedimentological details, petrography, and compositions of both clastic and carbonate sedimentary rocks are observed in the hand-specimen samples and thin-sections under polarizing microscope. The Phra That Formation composes of greenish-grey to reddish-brown sandstones, siltstones, conglomerates, and volcaniclastic rocks. These sedimentary successions are deposited in a near-shore shallow marine environment based on the facies analysis and sedimentological and petrographic studies. In addition, the fossils of bivalves, including Eumorphotis multiformis and Costatoria, brachiopods, and crinoid stem support the shallow marine environment interpretation. The Pha Kan and Doi Long Formations of which distribution is narrow and elongate, mainly consist of limestone, dolomitic limestone, argillaceous limestone, shale, and mudstone with minor intercalated calcareous sandstone. The shallow water, ramp carbonate platform is proposed to be the depositional environment of the Pha Kan Formation based on the stratigraphic study, grain types, microfacies distribution, compositions, and biotic compositions. The Doi Long Formation is interpreted as a product of shallow water reef or bank settings with a small-scale patch reef or small-sized bioherm productivity. The Hong Hoi Formation mostly consists of mudstone, sandstone, and shale with minor intercalated conglomerate and limestone. Based on lithofacies analysis, stratigraphic study, distribution, bed geometry, sedimentary structures, and fossils, the paleoenvironment of the Hong Hoi is interpreted to deposit in a submarine fan environment. The transportation patterns broadly represent a variety of sedimentary processes from the source areas, including debris flow, and high- to low-density turbidity current. Sandstones are normally graded and fining-upward depositional cycles, and with erosive base. Detailed paleocurrent analysis of sedimentary structures reveals three dominant flow directions including northeast toward southwest, southeast toward northwest, and northwest toward southeast depend on the locations of source rock. The results of paleocurrent directions suggest that the basin was transversely fed from the source areas. The petrographic study of the thin-sections of the conglomerate and sandstone samples of the Phra That and Hong Hoi Formations under the polarizing microscope show that the sandstones are poorly sorted, and grains are subangular and have medium sphericity. The sandstone samples are classified as lithic arenite and lithic greywacke. The results of provenance in the Phra That Formation suggested that the sediments were originated from an active magmatic arc and sedimentary basement uplift. Provenance analysis in the Hong Hoi Formation indicates that the source of sediments is transitional and dissected arc and from the reworked shallow marine limestone of the Pha Kan Formation. However, sandstone samples of the Pha Daeng and Wang Chin Formations are characterized by an abundant supply of quartz grains more than other formations, which suggests the influence of continental materials from the Indochina Block. In addition, the presence of mixed siliciclastic and carbonate sediments in the Pha Kan and Doi Long Formations suggests the influence of volcanic activity during the carbonate deposition. The sedimentation history of the Lampang Group in the Lampang Sub-basin, prior to the basin development, volcanic, pyroclastic, and volcaniclastic rocks of the PermoTriassic (PTrv) and Permian sedimentary rocks were existing in the Lampang area. The basin was initially open and uplifted. Erosion along the basin margin, initiation and transportation of volcanic-derived clastic sediment debris flow, and the deposition of basal limestone conglomerate occur during this period. The Phra That Formation is deposited in a near-shore, shallow marine basin in the Early Triassic. The shallow marine carbonate rocks of the Pha Kan Formation is conformably and unconformably overlying the Phra That Formation and is overlain by the Hong Hoi Formation. In the Middle Triassic, deep marine mudstone and sandstone sequences of the Hong Hoi Formation are widely distributed in the basin. The fauna fossils including bivalves (Daonella, Posidonia, and Halobia) and ammonoids (Paratrachyceras and Trachyceras) infer that the formation developed in a deep marine environment. Carbonate sedimentation of the Doi Long Formation is conformably overlying the Hong Hoi Formation in the Late Triassic due to the sea-level fall, less subsidence, and low amount of siliciclastic supply. The carbonate rocks of the Doi Long Formation are distributed in a narrow zone as belts in reef or bank settings. The transition from the Doi Long to the Pha Daeng Formations is erosional unconformity characterized by the presence of the basal limestone conglomerate, deposited at base of the Pha Daeng Formation. This evidence suggests a short period of uplifting of the shallow water carbonate of the Doi Long Formation. The fan-delta environment of the Pha Daeng Formation is characterized by the occurrence of shallow marine clastic depositions. During the Late Triassic, the basin was diminished probably due to uplifting because of the collision between the Sibumasu and Indochina Blocks.