Analysis of vertical position relationships between igneous sills and an unconformity surface — Interpretation of seismic profiles from the Northern Tarim Basin, NW China

Abstract

Sill emplacement mechanisms are very complex, diverse, and regional, and insights from sill reflections are helpful for understanding the emplacement process of magma in the Tarim Basin. This study takes advantage of high-quality 2D seismic data, which are rarely used to study sills in the Tarim Basin, to analyze the sills’ geometric characteristics, plan-view distributions, emplacement timing, and emplacement mechanisms with unconformity surfaces. In the seismic-reflection profiles of the middle-upper Ordovician in the North depression and the southern part of the Tabei uplift in the Tarim Basin, sills with strong positive polarity reflections appear, and they are closely distributed near the Tg52 unconformity surface, which represents the interface between Middle Ordovician limestone and Upper Ordovician mudstone. According to the vertical position of the sills relative to the unconformity, we can divide the sills into saucer-shaped or quasi-saucer-shaped sills above the unconformity surface, sill complexes and saucer-shaped sills on the unconformity surface, and saucer-shaped sills below the unconformity surface. Potential hydrothermal vents and peripheral faults associated with sill intrusion terminate upward in the Middle Permian strata, suggesting that these sills formed in the Middle Permian. Sills with inner flat sheets on the Tg52 unconformity surface formed when the magma ascended and encountered an abrupt change in the fracture toughness and tensile strength between the two adjacent host rock layers. The sills above and on the Tg52 unconformity surface overlap or are vertically linked; therefore, the sills above the Tg52 unconformity surface are the result of the continuous upward expansion of the sills on the unconformity surface, forming sill complexes. Our findings further confirm that unconformities are important interfaces that affect the emplacement of sills.