贵州台江八郎凯里组层序地层及海平面变化初探

贵州台江八郎凯里组以含钙质泥岩—泥灰岩、含钙质泥岩—粉砂岩和含钙质泥岩—灰岩等3种韵律性沉积为主,沉积体系域类型主要包含有大陆边缘体系域、海侵体系域和早期高水位体系域。在凯里组沉积初期,台江地区的水体相对较浅、生物相对较少;随着海平面的上升,生物在分异度和丰度上的大大增加,构成了台江生物群;当海平面升至最高点时,台江沉积区处于一个相对深水的环境,以大量的浮游类和较强游泳能力的生物为特征,成为了凯里生物群的开始;海平面逐渐下降,浮游生物逐渐减少;至凯里组沉积晚期,海平面的下降使台江地区的水体再度变浅、生物化石相对稀少。在台江八郎剖面上,凝缩段之下早寒武世的三叶虫Bathynotus消失,Orycto-cephalusindicus等三叶虫在凝缩段底界出现。凝缩段底界与下、中寒武统界线基本一致,表明了凝缩段是该区下、中寒武统地层划分、对比的关键。

DISCUSSION ON THE SEQUENCE STRATIGRAPHY AND SEA-LEVEL CHANGES OF THE KAILI FORMATION AT BALANG, TAIJIANG, GUIZHOU

The Kaili Formation at Balang Village, with the Kaili biota as one of the Burgess shale-type biotas, has been intensively studied in many respects, but little has been reported about sequence stratigraphy. In the Kaili Formation at Balang, the sedimentary rhythms are evidently composed of carbonate mudstone to marlite, or carbonate mudstone to siltstone, or carbonate mudstone to limestone. The Kaili Formation contains mainly shelf systems tract, transgressive systems tract, condensed section and early highstand system tract. In the early depositionary period of the Kaili Formation, the water in the Taijiang area was comparatively shallow, and there were few life-forms. When the sea-level rose, the kind and number of living things began to increase and then constituted the Taijiang biota. When the sea-level reached the top-level, the Taijiang area was in the environment of deeper water, and a large number of floating and swimming organisms, such as medusiform animals Pararotadiscus and bivalve arthropods Tuzoia and trilobites Pagetia, Oryctocephalus, were living here and constituted the beginning of the Kaili biota. Then, the floating organisms gradually decreased when the sea-level was descending. In the later depositionary period of the Kaili Formation, the sea-level was continuously descending so that the water in the Taijiang area gradually shoaled again, and there were few fossils. In the Balang section, the bottom of the condensed section is just consistent with the boundary of the Late-Middle Cambrian. The Early Cambrian trilobites, such as Redlichia, Protoryctocephalus, Bathynotus and others, disappear below the condense section, and the trilobites of Middle Cambrian age, such as Oryctocephalus, Xingrenaspis and others began to appear in the condensed section.