South-southwestward palaeocurrent swerved to east-southeast and then broadly to southeast over the transition from alluvial fan to axial channel and then to the flood plain in the Mio-Pliocene foreland system within w...South-southwestward palaeocurrent swerved to east-southeast and then broadly to southeast over the transition from alluvial fan to axial channel and then to the flood plain in the Mio-Pliocene foreland system within which the Siwalik Group depositd in Darjeel- ing-Jalpaiguri Districts, eastern India. Palaeocurrent pattern is found to be multi-modal on the fans, virtually unimodal on the axial channel zone and again multi-modal, more profoundly, on the flood plain. Coarse siliciclastic mass-flows were progressively eliminated and gave way to predominant bed-load transport downfan and the axial river, and then to suspensionload dominance in fine siliciclastics on the flood plain. Distal flood plain lacustrine sediment included most of the coals and the entire bulk of the dolomitic limestone. Further resolution in palaeogeography within the frame of aforementioned foursome facies associations is elicited in twenty-six distinctive facies altogether. Critical evaluation of chemical indices （CIA, CIW, ICV, PIA, as well as Rb/Sr ratio） for weathering and depleted （3180 values indicate a high precipitation rate. The contention is further corroborated by the high discharge rate calculated from cross-set thicknesses within the main channel deposits. Reconciliation of various relevant data sets collected or calculated from all known worksites along the entire 2000 km-Iong exposure belt of the Siwaliks along the Himalayan foothills reveal confluence of two tributaries, one from the west and the other from the east, close to the present study area before escaping onto the Indian plains. Channel parameters, channel-belt width and discharge thus attained maxima in the present study area. The precipitation rate and temperature increased eastward overall as a prelude to the modern trend in this regard. 13C enrichment indicates that the transition from C3 to C4 vegetation had already set in.展开更多
The Baicheng （拜城） piggy-back basin was part of the intracontinental foreland basin system of southern Tianshan （天山） Mountains. It was formed on a thrust-and-fold belt induced by Mio-Pliocene collision between ...The Baicheng （拜城） piggy-back basin was part of the intracontinental foreland basin system of southern Tianshan （天山） Mountains. It was formed on a thrust-and-fold belt induced by Mio-Pliocene collision between southern Tianshan Mountains and Tarim craton that controlled the thick synorogenic basin fills. Sedimentological analysis and a restored cross-section based on seismic data and field studies revealed three tectono-depositional sequences of synorogenic basin fills. （1） The Lower Miocene sequence （Jidike （吉迪克 ） Formation） was formed under alluvial-braided river-lacustrine environments, in response to geodynamic changes of the Kuqa （库车） fold and thrust belt from the embryonic foreland to a foredeep in the course of orogenic loading period. （2） The Upper Miocene sequence （Kangcun （康村） Formation） was developed in fluvial-delta and lacustrine environments, within a foredeep due to orogenic thrust. （3） The Pliocene sequence （Kuqa Formation） was formed in the Baicheng piggy-back basin that became a wedge-top depozone, thrusting in the Qiulitagh （秋里塔格） thrust belt that propagated progressively southward to the Yaken （牙肯） thrust belt.展开更多
基金Sunipa Mandal acknowledges to UGC, Minor Research Project, Government of India (F.PSW-032/08-09_ERO) for financial supportfinancial help from DST (ESS/16/243/2006-EMR-II)financial help from UPE-II programme of Jadavpur University
文摘South-southwestward palaeocurrent swerved to east-southeast and then broadly to southeast over the transition from alluvial fan to axial channel and then to the flood plain in the Mio-Pliocene foreland system within which the Siwalik Group depositd in Darjeel- ing-Jalpaiguri Districts, eastern India. Palaeocurrent pattern is found to be multi-modal on the fans, virtually unimodal on the axial channel zone and again multi-modal, more profoundly, on the flood plain. Coarse siliciclastic mass-flows were progressively eliminated and gave way to predominant bed-load transport downfan and the axial river, and then to suspensionload dominance in fine siliciclastics on the flood plain. Distal flood plain lacustrine sediment included most of the coals and the entire bulk of the dolomitic limestone. Further resolution in palaeogeography within the frame of aforementioned foursome facies associations is elicited in twenty-six distinctive facies altogether. Critical evaluation of chemical indices （CIA, CIW, ICV, PIA, as well as Rb/Sr ratio） for weathering and depleted （3180 values indicate a high precipitation rate. The contention is further corroborated by the high discharge rate calculated from cross-set thicknesses within the main channel deposits. Reconciliation of various relevant data sets collected or calculated from all known worksites along the entire 2000 km-Iong exposure belt of the Siwaliks along the Himalayan foothills reveal confluence of two tributaries, one from the west and the other from the east, close to the present study area before escaping onto the Indian plains. Channel parameters, channel-belt width and discharge thus attained maxima in the present study area. The precipitation rate and temperature increased eastward overall as a prelude to the modern trend in this regard. 13C enrichment indicates that the transition from C3 to C4 vegetation had already set in.
基金supported by the State Key Scientific and Technological Research Project (No. 2001BA605A-02-03-03-04)
文摘The Baicheng （拜城） piggy-back basin was part of the intracontinental foreland basin system of southern Tianshan （天山） Mountains. It was formed on a thrust-and-fold belt induced by Mio-Pliocene collision between southern Tianshan Mountains and Tarim craton that controlled the thick synorogenic basin fills. Sedimentological analysis and a restored cross-section based on seismic data and field studies revealed three tectono-depositional sequences of synorogenic basin fills. （1） The Lower Miocene sequence （Jidike （吉迪克 ） Formation） was formed under alluvial-braided river-lacustrine environments, in response to geodynamic changes of the Kuqa （库车） fold and thrust belt from the embryonic foreland to a foredeep in the course of orogenic loading period. （2） The Upper Miocene sequence （Kangcun （康村） Formation） was developed in fluvial-delta and lacustrine environments, within a foredeep due to orogenic thrust. （3） The Pliocene sequence （Kuqa Formation） was formed in the Baicheng piggy-back basin that became a wedge-top depozone, thrusting in the Qiulitagh （秋里塔格） thrust belt that propagated progressively southward to the Yaken （牙肯） thrust belt.
