30 January 2014
One of the intriguing rocks you find in Virginia, at the interface between the Valley and Ridge province and the Blue Ridge province, is distinctive brecciated Antietam Formation. The Antietam (sometimes known as the “Erwin,” especially in Shenandoah National Park), is a quartz arenite (quartz sandstone) that has been variably fused to quartzite in some places (but not others). It’s been deformed, sometimes spectacularly so, as we see when the individual grains have been (ductilely) stretched out into little blimp shapes, all aligned due to pressure solution.
In other places, it’s seen to have experienced a different kind of deformation, a brittle sort. In places, the Antietam is shattered, and then the resulting shards have been stuck back together into solid rock. This turns it into a breccia. Sometimes the breccia is cemented by quartz (same color as the clasts), and sometimes it’s stuck together with iron and manganese oxide minerals (which are dark brown or blackish). The latter is a high-contrast beauty. Here, you can examine this intriguing rock in outcrop and in hand sample via the magic of GigaPans:
How did this breccia form? There are many kinds. This one is neither a sedimentary breccia (for instance, made from sediment that accumulates at the base of a cliff), nor a volcanic breccia (which is lithified lahar deposits or pyroclastic surge deposits). One explanation is that it’s a tectonic breccia, the crushed up rock that formed along a fault zone. The contact between the Blue Ridge and the Valley & Ridge is indeed a thrust fault in some places. However, this outcrop and this sample, from the Boyce quadrangle, are from west of the floodplain of the Shenandoah River, in a section of the quad where there is no missing stratigraphic section – in other words, the “stack” of Blue Ridge layers goes “seamlessly” up (west) into the strata of the Valley & Ridge. This map pattern suggests there is no “need” for a fault to be postulated in this particular area. Another possibility is that this is a collapse breccia, wherein an adjacent carbonate unit dissolved away, then the Antietam broke into that void, shattering and scattering, and later fluid flow filled in the gaps between the pieces. Maybe it’s a dialational breccia, wherein a jog in the stress field ripped open a low-pressure area during Alleghanian deformation, and *kapow* the Antietam’s brittle bits exploded into that pocket.
Or some combination of all of these? Take a moment to explore these images (the deeper you go, the more detail you get), and let me know what you think.
Food for thought – some previous posts on this blog that discuss brecciation in the Antietam: