THE SEDIMENTARY SEQUENCE AND PALEOCLIMATIC-PALAEOGEOGRAPHY OF THE MIDDLE-LATE TRIASSIC ON NORTHWEST SHELF, AUSTRALIA
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摘要: 澳大利亚西北大陆架中上三叠统中发现多个世界级大气田,其地层发育完整,厚度大且分布广,沉积特征典型:(1)平面上以浅水辫状河三角洲平原亚相为主,三角洲前缘相带不发育;(2)垂向上具有"三明治"式沉积特点,平原相带厚层砂岩与暗色泥岩、薄煤层互层,其中分支流河道砂岩粒度较粗,含大量炭屑,是有利的储集砂体,分支流河道间湾泥有机质含量高,是良好的气源岩;(3)孢粉组合上,近岸低地种子蕨、蕨类孢粉与高山松科孢粉共存。这类典型的沉积组合体现了当时特殊的古气候-古地理背景:三叠纪澳大利亚西北大陆架处于稳定的澳洲-南极克拉通北缘的浅海背景下,受到外围岛屿对海水的屏蔽作用,沉积作用以河流作用为主;澳洲-南极古陆以元古代-太古代花岗岩为主,为澳大利亚西北大陆架提供了丰富的粗碎屑物源;中晚三叠世澳大利亚西北大陆架为热带-亚热带气候带,受到环特提斯洋季风的影响大,气候温暖潮湿,植被茂密,洪泛作用频繁,导致粗砂岩与暗色泥岩互层、近海低地孢粉与高山孢粉混合共存。Abstract: Some world-class gas fields have been discovered in the Middle to Upper Triassic, which is complete and widely distributed, on the Northwest Shelf of Australia. The typical sedimentary sequence is characterized by:(1) The shallow braided stream delta is the commonest depositional system which consists of a broad deltaic plain and narrow deltaic front; (2) The sedimentary sequence of the shallow braided stream delta shows a "sandwich" structure by the alternation of the distributary sandstone and the dark interdistributary bay mudstone. The distributary sandstone which contains large amount of carbonaceous laminations, is medium-grained up to coarse-grained and could be a kind of reservoir with high quality, and the interdistributary mudstone rich in terrigenous organic matter is a kind of good gas source rock; (3) The seed ferns and fern spores from the nearshore lowland is always mixed with Alpine pine pollen from the mountain. The typical sedimentary assemblage mentioned above is the response to the specific palaeoclimate and paleogeography. The Triassic occurs upon a shallow marine background on the northern margin of the stable Australian-Antarctic craton. Because of the shielding of the outside islands, current process is the main depositional process of the Triassic delta. The Australia-Antarctic ancient land mainly composed of Proterozoic-Archean granites provided a large amount of coarse clastics for the Middle to Upper Triassic delta. In the Middle-Late Triassic, Australian North West Shelf lain in the tropical -subtropical climate zone and was influenced by the Central Tethys Ocean monsoon. The climate was warm and humid, the vegetation was flourishing, and flood was frequent. The coarse sandstones formed during the flood period were interbeded with dark mudstone formed during the flood interval. Offshore lowland spore-pollen are always mixed with Alpine spore-pollen.
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Key words:
- paleoclimate /
- palaeogeography /
- Australian Northwest Shelf /
- sedimentary sequence /
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