Title : Anisotropy of magnetic susceptibility in sand injectites from the San Joaquin Basin, California (EUA)
Abstract:
Sand extrudites represent the shallowest genetic and architectural elements of sand injectite systems. They form by the extrusion of fluidized, overpressured sand at the surface, commonly fed by an underlying intrusive network of dykes and other sandstone intrusions. Because they may preserve sedimentary structures related to sediment dispersion and palaeoflow, sand extrudites can provide important constraints on the timing, dynamics, and architecture of sand injection complexes. In hydrocarbon-bearing basins, sand injectite systems are relevant because they may increase reservoir volume, enhance connectivity, or act as pathways for fluid leakage. Despite their significance, sand extrudites remain relatively underdocumented in both outcrop and subsurface studies. A better understanding of their internal fabric and emplacement processes is therefore important for sedimentary basin analysis and reservoir characterization. This study applies anisotropy of magnetic susceptibility (AMS) and rock magnetic methods to sand extrudite samples from the shallowest part of the Panoche Giant Injection Complex, developed within the upper Moreno Formation in the San Joaquin Basin, California. AMS data were integrated with thermomagnetic curves, hysteresis loops, and isothermal remanent magnetization experiments to characterize magnetic fabric orientations and identify the main magnetic mineral carriers.
The results indicate complex magnetic properties and fabric patterns within the studied deposits. Adjacent mudstones are characterized mainly by sub-horizontal magnetic foliations, whereas sandstone units show oblique to sub-horizontal fabrics. Bodies interpreted as sand volcanoes or vent-fill structures display steep to high-angle magnetic foliations in their central portions, associated with intensely deformed zones containing convolute and flame structures. Toward the margins, foliation dips become progressively shallower, broadly consistent with lateral sediment dispersion and semi-radial palaeoflow patterns. Rock magnetic data indicate that magnetite and titanomagnetite are the main magnetic carriers, with predominantly low coercivity and pseudo-single-domain behavior. These results show that AMS, when combined with rock magnetic analyses and field observations, can provide useful constraints on the internal architecture and emplacement dynamics of sand extrudites. The approach contributes to the interpretation of sand injectite systems and their potential role in reservoir architecture, connectivity, and fluid migration pathways.
