Limestone breccias are a common phenomenon in the Cambrian successions worldwide. They bear important geological im- plications that have attracted geologists for several decades. There are, however, still controversi...Limestone breccias are a common phenomenon in the Cambrian successions worldwide. They bear important geological im- plications that have attracted geologists for several decades. There are, however, still controversies on their origins, especially those of the breccias with abundant vertically orientated clasts. The Furongian (upper Cambrian) Chaomidian Formation of the North China Platform contains numerous levels of limestone breccias and conglomerates that provide an excellent example to look into their formative processes. These breccias and conglomerates have been the focus of study and discussion since the 1980s, but yet there is still no consensus with respect to their geneses. Recently, Van Loon and others argued that the vertically orientated clasts of the breccias developed by a number of simultaneous "fountains" on the paleo-seafloor; the "fountains" formed by upward-directed fluidized flows originated from the sediment underlying the brecciated limestones. While the novel "fountain" hypothesis is not impossible, based on field evidences and theoretical considerations, however, it is most likely that the vertically orientated clasts resulted from their re-orientation by gillaceous sediment that was interbedded with brecciated limestone took place under shallow burial. upward flow of thixotropically liquidized, uncemented ar- fragments. Besides, the deformation processes most likely展开更多
The Longmenshan thrust belt(LMTB) is one of the best natural laboratories for thin-skinned tectonics and has developed a series of NE-SW trending fold-and-thrust structures represented by a series of nappes and klippe...The Longmenshan thrust belt(LMTB) is one of the best natural laboratories for thin-skinned tectonics and has developed a series of NE-SW trending fold-and-thrust structures represented by a series of nappes and klippes, exemplified by the Tangbazi and Bailuding klippe. However, the timing and emplacement mechanism of these klippes are still in dispute. Three possible mechanisms have been proposed:(1) a Mesozoic-Cenozoic southeastward thrusting,(2) a Cenozoic gravity gliding, and(3) glacial deposition. Almost all of these klippes are tectonic and overlaid on folded Late Triassic sandstone except the Tangbazi klippe, which is located in the center of the LMTB and has a narrow tail extending southeastward and covering Jurassic-Quaternary rocks. This geometric relationship is considered the most important stratigraphic evidence to support the post-Cenozoic emplacement of the Longmenshan klippe. Our structural and petrological observations show that the rocks at the front of the Tangbazi and Bailuding structures are brecciated limestone, which is assumed to have been generated by a gravitational collapse and is not characteristic of the massive Permian strata. Artemisia pollen, which has been exclusively recognized in post-Late Eocene strata in Central Asia, was found in the matrix of this brecciated limestone. Therefore, our discovery indicates that the brecciated limestone was deposited after the Late Eocene rather than during the Permian as annotated on the geological map. In contrast, unbrecciated, massive Permian limestone overlaid on the folded Late Triassic rocks. Hence, the anomalous relationship of Permian strata overlaying Late Triassic rocks cannot be evidence of Cenozoic emplacement. According to currently recognized bulk strata relationships, we can only be sure that the klippe was emplaced in the post Late Triassic. The petrological characteristics of the brecciated limestone show that it was crumbled before the re-sedimentation of the breccia, implying that the LMTB might have experienced a rapid uplift during the Late Eocene.展开更多
基金supported by the National Research Foundation of Korea(Grant No.2008-0093871)the National Natural Science Foundation of China(Grant Nos.41302077,41290260)
文摘Limestone breccias are a common phenomenon in the Cambrian successions worldwide. They bear important geological im- plications that have attracted geologists for several decades. There are, however, still controversies on their origins, especially those of the breccias with abundant vertically orientated clasts. The Furongian (upper Cambrian) Chaomidian Formation of the North China Platform contains numerous levels of limestone breccias and conglomerates that provide an excellent example to look into their formative processes. These breccias and conglomerates have been the focus of study and discussion since the 1980s, but yet there is still no consensus with respect to their geneses. Recently, Van Loon and others argued that the vertically orientated clasts of the breccias developed by a number of simultaneous "fountains" on the paleo-seafloor; the "fountains" formed by upward-directed fluidized flows originated from the sediment underlying the brecciated limestones. While the novel "fountain" hypothesis is not impossible, based on field evidences and theoretical considerations, however, it is most likely that the vertically orientated clasts resulted from their re-orientation by gillaceous sediment that was interbedded with brecciated limestone took place under shallow burial. upward flow of thixotropically liquidized, uncemented ar- fragments. Besides, the deformation processes most likely
基金the National Natural Science Foundation of China (Grant Nos. 41372028, 41225009 & 41472193)the Project of Major State Special Research on Petroleum (Grant No. 2011ZX05008-001)
文摘The Longmenshan thrust belt(LMTB) is one of the best natural laboratories for thin-skinned tectonics and has developed a series of NE-SW trending fold-and-thrust structures represented by a series of nappes and klippes, exemplified by the Tangbazi and Bailuding klippe. However, the timing and emplacement mechanism of these klippes are still in dispute. Three possible mechanisms have been proposed:(1) a Mesozoic-Cenozoic southeastward thrusting,(2) a Cenozoic gravity gliding, and(3) glacial deposition. Almost all of these klippes are tectonic and overlaid on folded Late Triassic sandstone except the Tangbazi klippe, which is located in the center of the LMTB and has a narrow tail extending southeastward and covering Jurassic-Quaternary rocks. This geometric relationship is considered the most important stratigraphic evidence to support the post-Cenozoic emplacement of the Longmenshan klippe. Our structural and petrological observations show that the rocks at the front of the Tangbazi and Bailuding structures are brecciated limestone, which is assumed to have been generated by a gravitational collapse and is not characteristic of the massive Permian strata. Artemisia pollen, which has been exclusively recognized in post-Late Eocene strata in Central Asia, was found in the matrix of this brecciated limestone. Therefore, our discovery indicates that the brecciated limestone was deposited after the Late Eocene rather than during the Permian as annotated on the geological map. In contrast, unbrecciated, massive Permian limestone overlaid on the folded Late Triassic rocks. Hence, the anomalous relationship of Permian strata overlaying Late Triassic rocks cannot be evidence of Cenozoic emplacement. According to currently recognized bulk strata relationships, we can only be sure that the klippe was emplaced in the post Late Triassic. The petrological characteristics of the brecciated limestone show that it was crumbled before the re-sedimentation of the breccia, implying that the LMTB might have experienced a rapid uplift during the Late Eocene.