The extent of disequilibrium between certain members of the naturally occurring radioactive heavy element families in recent and fossil marine molluscan shells has been investigated with a view to ascertaining the conditions under which these disequilibria may serve as measures of geologic age. The results obtained lead to the following conclusions. The average uranium concentration in fossil marine shells is generally higher than in modern shells from the same region. Greater than equilibrium quantities of Ra226 are present in most modern marine shells. The predominant thorium alpha activity in shells of living marine mollusks is associated with Th228, which presumably arises from decay of Ra228 in the specimen. The Th230 concentration in modern marine shells is near zero and is probably less than 1% of the equilibrium quantity. Uranium and thorium compositions of fossil marine molluscan shells collected from terrestrial environments have generally been altered by contact with ground waters. For this reason, age determination of fossil marine carbonates by uranium decay series disequilibrium measurements, although apparently sometimes applicable to fossil corals and other fossil marine carbonates that have formed and have remained in a marine environment, is not likely to give reliable results when applied to isolated fossil marine shell specimens collected from terrestrial environments.

On the basis of reprocessing 34 new two‐dimensional spliced long sections (20,191 km) in the Tarim Basin, the deep structure features of the Tarim Basin were analyzed through interpreting 30,451 km of two‐dimensional seismic data and compiling basic maps. Seismic interpretation and geological analysis conclude that the Nanhua‐Sinian strata are a set of rift‐depression depositional systems according to their tectonic and depositional features. The rift valley formed in the Nanhua Period, and the transformation became weaker during the late Sinian Period, which eventually turned into depression. From bottom to top, the deposited strata include mafic igneous, tillite, mudstone, and dolomite. Three major depocenters developed inside this basin during the rift stage and are distributed in the eastern Tarim Basin, the Awati area, and the southwestern Tarim Basin. Among them, the rift in the eastern Tarim Basin strikes in the near east‐west direction on the plane and coincides with the aeromagnetic anomaly belt. This represents a strong magnetic zone formed by upwelling basic volcanic rock along high, steep normal faults of the Nanhua Period. Controlled by the tectonic background, two types of sedimentary systems were developed in the rift stage and depression stage, showing two types of sequence features in the Sinian depositional stage. The Nanhua System appears as a wedge‐shaped formation, with its bottom in unconformable contact with the base. The rifting event has a strong influence on the current tectonic units in the Tarim Basin, and affects the distribution of source rock in the Yuertus Formation and reservoir beds in the Xiaoerbulake Formation in Lower Cambrian, as well as the gypseous cap rock in Middle Cambrian. The distribution features of the rifts have important and realistic significance for determining the direction of oil and gas exploration in the deep strata of the Tarim Basin. Comprehensive analysis suggests that the Tazhong region is the most favorable zone, and the Kalpin‐Bachu region is the optimal potential zone for exploring sub‐salt oil and gas in deep Cambrian strata.