Composition and Provenance of Sandstones and Siltstones in Paleogene,Huimin Depression,Bohai Bay Basin,Eastern China

This study was conducted to distinguish the compositions and provenance of sandstones and siltstones in the Ek1-Es3 members of Huimin （惠民） depression. The samples have been analyzed for petrographic, major element and selected trace element compositions （including REE）. The results show that the sandstones from the first member of Kongdian （孔店） Formation （Ek1） have higher quartz compositions than those from the fourth member （Es4） and the third member （Es3） of Shahejie （沙河街） Formation. The alkali feldspar/plagioclase ratio in the A-CN-K diagrams decreases in the order Ek1〉Es4〉Es3, which suggests that the Ek1 member had a more alkali feldspar-rich granitoid source and more intense weathering of the source than the Es4 and Es3 members. The mineral distributions in the A-CN-K diagrams also indicate that the sandstones and siltstones in the three members underwent K-metasomatism. Extrapolation of the sandstones and siltstones back to the plagioclase-alkali feldspar line in the A-CN-K diagram suggests a high average plagioclase to alkali feldspar ratio in the provenance （tonalite to granite）. In addition, the chemical index of weathering （CIW） and chemical index of alteration （CIA） parameters of the sandstones and siltstones suggest that the weathering of the first cycle material was intense, and the CIW decreases in the order Ek1〉Es4〉Es3 Trace element ratios suggest all the sedimentary rocks were mostly derived from granitoids. Elemental ratio plots （e.g., Th/Sc vs. Eu/Eu^＊） of sandstones and siltstones suggest a mix of a granodiorite-tonalite source as a source of the sandstones and siltstones. The sandstones and siltstones of Es3 and Es4 members also contain higher Eu/Eu＊ and lower Th/Sc ratios than the sandstones and siltstones of Ek1 member. As is consistent with the petrography, it suggests that the sandstones and siltstones of Es3 and Es4 members are derived from a source with less granite and more granodiorite-tonalite than the sandstones and siltstones of Ek1 member.

Tectonics and Mechanisms of Uplift in the Central Uplift Belt of the Huimin Depression

The Huimin（惠民） depression is a third-level tectonic element of the Bohai（渤海） Bay basin in eastern China.The central uplift belt is the most important oil and gas accumulation zone in the depression,but the lack of adequate geological studies in the area has greatly hindered exploration and development.In this article,using seismic data,fracture mechanics,and a combination of data on fault growth indices and fault throws,we present an analysis of tectonic activity in the central uplift belt and adjacent regions.The amount of extension is calculated along balanced N-S cross-sections,along with the thickness of strata eroded from the fourth,third,and second members of the Shahejie（沙河街） Formation（Es4-Es2） in the uplift belt,by analyzing porosity and stratigraphic correlations.In addition,uplift features are described,and their timing and processes of formation are analyzed and dis-cussed.The results indicate that strike-slip and extensional tectonic movements coexisted,with the effects of the latter most obvious.The spatial and temporal nature of the extensional move-ments is varied：fault activity during the period Ek-Es4 was the strongest on the northern Ningnan（宁南） fault,and activity in the western part of the area was stronger than that in the east,which in turn was stronger than that in the central region;during Es3-Es2,the strongest fault movements were along the eastern part of the middle Linyi（临邑） fault,and activity in the western part of the area was rela-tively weak,whereas in the mid-west it was the strongest.The extensional movements were a response to the activity of the faults.The sediments in the lower part of the fourth member of the Shahejie For-mation（Es4x） show that Es4 was the time when the central belt first began to be uplifted strongly.Uplift was uneven during the Paleogene：the western part of the area was uplifted continuously,while the mid-eastern area underwent alternating periods of uplift and subsidence.During the Paleogene,a number of different tectonic features developed in the central uplift belt at different times and are manifested as follows：during the period Ek-Es4,a gentle slope was formed as part of the Zizhen（滋镇） sag;during Es3-Es2,the northern part of the central uplift belt continued to display a gentle slope as part of the Zizhen sag,but the southern region developed an steep slope as part of the Linnan（临南） sag.There are close interrelationships between uplift,strike-slip,and extension within the central uplift belt of the Huimin depression,as is manifest by the areas of strongest extension being uplifted most rapidly,and also eroded the most.