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煤、碳质泥岩等含有大量碳质的沉积岩的特征,是热导率异常低(仅为其它一般岩类的1/5～1/10)。其结果是即使在平均热流的条件下,这类沉积岩中的地温梯度亦可达0.25°Cm~(-1)。当这样陡的温度梯度存在于一段很大的沉积厚度中时即会提高下伏岩石单元的温,使无机相和有机相都能记录到觉得异常高的热成熟度。这种碳质盖层的绝热效应有助于解释在阿帕拉契亚盆地,密彻安盆地,和世界上其它具有大量碳质地层的地区于浅部观测到的异常成熟度。 在许多沉积盆地的地表或浅部埋藏深度中都能观察到诸如由煤级,镜质体反射率,牙形石变质指数,裂变径迹衰退曲线,流体包裹体均一化所指示的高热成熟度。由于正常地温梯度不可能产生这些指标在其现时埋深上所反映的古温度,这样高的成熟度就称为异常成熟度。近来的许多研究曾把异常热成熟度的存在解释为与明显的高热流或大区域抬升和侵蚀等有关的古构造事件的证明。本文中我们提出另一种假说,把异常热成熟度简单地解释为只是由低热异率碳和沉积岩构成的一层较薄热盖层的结果。
The characteristics of sedimentary rocks containing large amounts of carbon such as coal and carbonaceous mudstone are exceptionally low thermal conductivity (only 1/5 ~ 1/10 of other general rocks). The result is a geothermal gradient of up to 0.25 ° Cm -1 in these sediments, even under mean heat flux. When such a steep temperature gradient is present over a large depositional thickness, the temperature of the underlying rock unit is raised, allowing both the inorganic and organic phases to record unusually high thermal maturities. The adiabatic effect of this carbonaceous cap helps to explain the abnormal maturity observed in the shallow parts of the Appalachian Basin, the Mightchean Basin, and other parts of the world with large amounts of carbonaceous strata. In many sedimentary basins, surface or shallow burial depths can be observed, such as by the coal grade, vitrinite reflectance, conodont deterioration index, fission track regression curve, homogenization of fluid inclusions indicate high thermal maturity . Because the normal temperature gradient can not produce these indicators reflect the current depth of its ancient temperature, such a high maturity is called abnormal maturity. Many recent studies have interpreted the existence of anomalous thermal maturity as proof of palaeoclimate events associated with significant high heat flux or uplift and erosion of large areas. In this paper, we propose another hypothesis that simply interprets the anomalous thermal maturity as a result of a thinner thermal cap comprised of low-thermal-albedo carbon and sedimentary rocks.