Provenance and Petrological Evolution of Sand Injection Complexes: Insights From Heavy Mineral Analysis in the Paleogene Forearc Succession of the San Joaquin Basin

The lithostratigraphy of sand injection complexes (SICs) is governed by the architectural and petrological relationships between depositional parent units and intrusive networks. During the formation and evolution of SICs, diverse geological processes can modify mineral assemblages and textures, offering opportunities to evaluate genetic mechanisms and support petrological correlations. This study integrates field mapping with heavy mineral analysis of the Paleogene succession in the San Joaquin Basin, specifically targeting the upper Eocene Tumey Giant Injection Complex (TGIC) to investigate provenance of depositional and intrusive sandstones and processes associated with injection emplacement. Statistical analysis of heavy mineral assemblages and provenance-sensitive indices (MZi, GZi, CZi and RZi) confirms the genetic link between the slope channel-fills of the Kreyenhagen Formation and sandstone intrusions. These assemblages indicate derivation from granitic and metasedimentary sources derived from the Sierra Nevada Province to the east. In contrast, the underlying shallow-marine Domengine Formation reveals mixed sourcing, with significant blueschist-facies minerals (lawsonite and glaucophane) likely supplied by the Franciscan Complex accretionary prism to the west. While low ZTR values in the parent channel-fills and most intrusions indicate mineralogical immaturity, specific intrusive facies exhibit high ZTR and low ATi, suggesting mechanical degradation of less durable grains during sand injection. Qualitative grain morphology analysis reinforces this, showing a higher degree of grain damage within intrusions in comparison to parent unit. Furthermore, density-controlled ZTi index reveals hydraulic segregation, evidenced by the progressive settling of denser zircon grains through a km-scale wing-like intrusion. The petrological signature of the TGIC is thus a function of parent unit composition, intra-granular mechanical interaction, and hydraulic sorting under dynamic fluid-flow conditions. Despite the inherent 3D architectural complexity of the intrusive networks, this study provides robust methodological tools for investigating the evolution of injectites in diverse geodynamic settings.