泰國近岸古環境Toarcian-Aalenian期風暴沉積初探

泰國近岸古環境Toarcian-Aalenian期風暴沉積初探

AIGNER T. 1985. Storm depositional systems: dynamic stratigraphy in modern and ancient shallow-marine sequences. Lecture Notes in the Earth Sciences, 3(8):174.

MEESOOK A, SHA J. 2010. The Jurassic System of Thailand.University of Science & Technology of China Press,125.

Toarcian-Aalenian Storm Deposits in Nearshore Palaeoenvironments of Thailand–preliminary Results of Case Study

Assanee MEESOOK1,2), Michal KROBICKI3,4)
1)53 Mu 5, Tambon Thakham, Samphran District, Nakhon Pathom Province73110,Thailand;
2)C/O Bureau of Fossil Protection, Department of Mineral Resources; Rama VI Road, Bangkok10400,Thailand;
3)Polish Geological Institute–National Research Institute, Upper Silesian Branch, Krolowej Jadwigi1, 41-200Sosnowiec;
4)AGH University of Science and Technology, Department of Geology, Geophysics and Environmental Protection,Mickiewicza30, 30-059Krakow

Storm Deposit; Jurassic; Malayan Peninsula; Thailand; Bivalve

10.3975/cagsb.2012.s1.21

Jurassic deposits of the Malayan Peninsula(Thailand) are stratigraphically limited to Toarcian and Aalenian only (Meesook & Sha, 2010, and literature cited therein). These are represented by wide spectrum of both clastic and carbonate rocks with full development of transitional ones. In the middle part of peninsula (vicinity of Krabi) distribution of Jurassic facies suggest occurrence of full marine palaeoenvironments in NE part of it with gentle transition by lagoonal(brackish?) and tidal environments up to terrestrial ones in the SW part of peninsula. Sections studied,close to Thung Song city, are represented by 20 m thick sequence of thin-bedded limestones and marls of the Khlong Min Formation, most probably lagoonal palaeoenvironments (transitional = restricted marine-brackish), indicated by specific benthic fauna dominated by different kind of ethological group of bivalves – infaunal, epifaunal and encrusted (epibiontic) species likeProtocardia,Praemytilus,Modiolus,Myrene, andActinostreon. Very characteristic is a lack of full marine, stenohaline, other invertebrates –brachiopods, crinoids, echinoids etc. Several beds manifested high hydrodynamic environments of deposition, presumable connected with repeated storm events (tempestites) (sensu Aigner, 1985) with characteristic features for them: sharp bases of beds, mass occurrence of bioclasts in the lower part of beds (especially large fragments of disarticulated shells of bivalves), gradation of these bioclasts upwards with transition to micritic limestones/marls in the uppermost part of such beds, and from time to time with hummocky cross stratification (HCS) in the same parts.Such type of beds repeated several times in studied sections and consist both by infaunal bivalves and other ethological groups. The first type of bivalves have been removed from fine-grained sediments(mainly calcareous mud-supported), crushed/fragmented of shells and sedimented far from life area with fractionation effect of their remains due to storm currents. Another type is mainly represented by small oysters (oyster coquinas), which might primarily build small patch-reefs, but destroyed completely during storm actions. Originally they could be some submarine morphological barriers between full marine environments and more protected lagoonal/brackish ones,as mentioned above. More detailed studies of these type deposits are necessary, including ethological, taphonomical and bio-astratinomical investigations and also geochemical analysis of isotopic contents (especially C and O) in their shells as good indicators of salinity regimes of the Jurassic palaeoenvironments of the Malayan Peninsula, according to wider Early/Middle Jurassic palaeogeographical context of this region.