Characters And Forming Conditions Of Burnt Rocks In Yan'an Formation Of Northern Ordos Basin

The spontaneous combustion of coal seams and burnt rocks are a common geologic feature in many countries throughout the world. Their study are beneficial to the discussion on the tectonic movement, paleoclimate and paleogeography that coal seams are subjected to after they were formed. The coal seams in Yan'an formation of Northern Ordos Basin beard spontaneous combustion universally during terrestrial time, that resulted the wide range of burnt rocks. The study of burnt rocks in this area is help to discussing the structural evolution characters of Ordos Basin.In this paper, the burnt rocks in Northern Ordos Basin are taken as the study object. First, the emergence condition of spontaneous combustion and the characters of burnt coals (seams) are discussed, base on those, the degree of combustibility of the coal seams in Northern Ordos are estimated. And then, according to the forming characteristic, the burnt rocks are divided into two series in the section: the melted rocks and the baked rocks. After that, the section distributing characters, physical properties, petrologic characters and geochemical characters, magnetism characters of burnt rocks are researched, using the methods of SEM, EDS, IR, XRD, archeomagnetic method and XRF, 1CP-MS. Through these studies, more comprehensive features of the burnt rocks are obtained, i.e., the burnt rocks usually distributed along the valley in plain, and have obviously zonality in section; about the physical properties, burnt rocks have obviously increased porosity and water content, decreased compressive strength and shearing strength, and their logs are different from that of the normal rocks; some new dehydrated minerals, e.g., anhydrite, anhydromica, kaolinite , and some magnetic minerals, e.g., hematite and magnetite, appeared in burnt rocks, besides, the primary minerals, e.g., quartz, feldspar and clay minerals, all show the obvious burnt and melted traces because of the high-temperature baking; the REE distribution patterns of burnt rocks are similar to that of sedimentary rocks, but the fractionation of REE occurred, resulting in the difference between melted rocks and baked rocks in the REE distribution pattern; the burnt rocks have remarkable enhanced magnetism relative to the normal sedimentary rocks, and their also show that the susceptibility anisotropy smaller as the burning degree increases.The age of the burnt rocks in study area is studied detailed. Using the method of apatite fission track, and combining the analysis of geological phenomena, it is thought that, the forming period of burnt rocks span a long time, from last stage of Late Crataceous to Quaternary, and with the characteristic of episodic burnt.Basing on the above finding about the characters and forming age, the burnt rocks in studying area are classified into three kinds, i.e., the opening type burnt, the enclosed type burnt and the superimposed type burnt. Every kind of burnt rocks' forming condition is analyzed detailed, and their relevant burnt mode are given. The further analysis reveal that, the episodic forming of the burnt rocks in studying area are closing relative to the background of basin's tectonic development since Late Crataceous, i.e., it is coincided with the episodic of the main part of the basin's uplifts continuously, episodically and differently, strong and uneven erosion. And the later is the regional dynamic setting of the superimposed type burnt rocks' forming in Northern Ordos Basin.Besides, a kind of special burnt rock section that is different from the normal burnt rock section is found, and it is analyzed exhaustively, that Ningtiaota section in Kaokaowusugou is taken as the sample.