The sandwich horizon is the (presumably) time-transgressive surface along which Upper Border Series roof rocks and Layered Series floor rocks finally met, as the last Skaergaard pluton liquid solidified. The horizon occurs, by definition between UZc and UBZγ. The McBirney (1989) geologic map shows the location of the sandwich horizon surrounding Basistoppen and on Nunatak 1. However, the three field trip leaders concurred that the sandwich horizon is very difficult to actually find in the field. As a residual liquid, the sandwich horizon should be extremely iron-rich, should have Na-rich feldspar, and probably should have abundant minerals rich in otherwise incompatible elements including, perhaps, zircon, K-feldspar, and biotite. We were shown two candidates, each favored by different people.
Basistoppen (885 m), photographed from a minor summit at 504 m, just west of Basistoppen. Our traverse took us around the right side of the summit cone, where the first sandwich horizon candidate was pointed out. The sandwich horizon is supposed to skirt around the lower slopes of this peak.
Outcrop of the possible sandwich horizon candidate 1, on the south side of Basistoppen. Trip leader Kent Brooks concluded that this was about as close as we could get to the sandwich horizon, which is very difficult to find in the field. Trip leader Neil Irvine said that this was probably the most likely sandwich horizon candidate he had (ever?) seen.
The sandwich horizon candidate 1 rock. This rock appeared to be outcrop, but I personally could not be certain that it wasn’t a large fallen block. The Basistoppen sheet is above this point, and some blocks from there were part of the local talus. However, they are gray-weathering, unlike the brown-weathering Skaergaard. Thin section photos here and here.
The east flank of Nunatak 1, showing the sandwich horizon candidate 2 rock. This was certainly solid outcrop. Trip leader Jens Andersen thought this was the best candidate for the sandwich horizon. This location is also just below the Basistoppen summit cone. Gabbrofjeld is the snowy peak in the distance.
Close-up of the sandwich horizon candidate 2 rock on the eastern flank of Nunatak 1, shown in the photo above. It’s a rather fine-grained gabbro, and has little in the way of preferred orientation of crystals.
Irregular mafic clots and more anorthositic patches in UBZγ, on top of Nunatak 1. It isn’t clear how these things formed. They don’t look like xenoliths, autoliths, deformed layers, or injected material, and may be a replacement. Another possibility is that they are related to unmixing of immiscible magmatic liquids, as summarized by Holness et al (2017). This whole outcrop was covered with these things, and looks very different from the other outcrop on the south side of Basistoppen.
Did we actually see the sandwich horizon? I’m not sure.
