A PROPOSED MODEL FOR THE ORIGIN OF BRECCIA IN ANTIETAM FORMATION, WESTERN BLUE RIDGE, VIRGINIA

A belt of breccia crops out in Antietam Formation quartzite in central Virginia’s Blue Ridge. Two varieties of matrix occur in the breccia: silica & iron/manganese oxides. A long-standing interpretation is that these breccias formed through crushing action from the northwestward transport of the Blue Ridge atop Valley & Ridge strata along a major thrust. Some breccias show textural characteristics which are inconsistent with faulting alone, including a matrix-supported texture where many large clasts “float”, separated by wide zones of matrix of either composition. We observed bedrock outcrops & collected samples at three locations. The textures of these samples were referenced to published breccia varieties. We also studied thin sections of these breccias in the collections of Dan Doctor, USGS, documenting key microtextures. We differentiate four different generations of matrix that correlate across samples. Ar/Ar dates of 36-2 Ma from cryptomelane in the most recent generation of Fe/Mn cements were published by Will Odom (2020): far more recent than Alleghanian faulting. We synthesize these various observations with a multistage model for the formation of various cements within these samples. The model involves both the grinding action of traditional faulting as well as low-pressure sites of implosion, where slight jogs along the fault trace opened rhombohedral zones of dilation. Clast-supported breccias are interpreted to have formed along the tabular stretches of the fault, while the matrix-supported texture is interpreted as having formed due to implosion in the rhombochasms. Initially, both textures were cemented by silica. Later, as uplift & erosion removed the overlying rock mass, the Antietam breccias got closer to Earth’s surface. Once within 100 meters of the surface, meteoric water replaced the silica cement with iron & manganese oxides. This happened at least twice, based on the evidence of multiple distinct iron/manganese oxide cements. Dating of the youngest cement constrains when the breccia rose to -100m. The initial hypothesis of these being fault breccias is therefore plausible, though a fuller picture has now emerged. We account for variation in texture due to changes in fault orientation & in cement via multiple cycles of fluid infiltration, dissolving & replacing breccia cements.