Autolith blocks

Photos ©Kurt Hollocher, 2001

Foundered roof blocks are found extensively in LZb, LZc, and in the MZ, and less commonly elsewhere. Blocks range from a few centimeters in size, scarcely larger than the host rock plagioclases, to giant blocks over 400 m across. The rock of these autoliths is generally more plagioclase-rich than the enclosing lower zone host rocks, suggesting that the roof region from which the blocks must be derived was less efficient at trapping the denser mafic minerals, or perhaps that the roof region was enriched in plagioclase crystals that preferentially escaped deposition on the pluton walls and floor. Because the upper border series (roof zone) compositional and mineralogical layering is essentially a mirror image of that of the layered series (floor zone), the foundered roof blocks can be assigned to a particular stratigraphic order. This concept predicts, and it is found to be true, that the foundered blocks always have mineralogy similar to the enclosing or older rocks in the stratigraphy. It is found that roof rocks can also contain blocks. These must have detached, been carried laterally, and entrapped in the progressively solidifying roof magma. Though I saw no upper zone rocks in place except a little UBZγ on Nunatak 1, one large foundered block we saw in the MZ on Kramer Island did itself contain blocks.

View to the northwest toward Wagers Top from the landing area on the shore of Uttental Sund. Visible below Wagers Top are the Triple Group layers, highlighted by snow, and the largest known autolith to the upper right of the glacier (for better photos see Irvine et al., 1998, Figures 14B and 20; Andersen et al., 1998, Figure 2; Irvine et al., 2001B, Plate 22A). On the flanks of Pukugagryggen, the mountain to the right, the Triple Group is visible near the upper right of the photo, and a bit lower down is a large autolith. Below the autolith is a thick, prominent macrorhythmic section that can barely be discerned also on Wagers Top where the beds lap up onto the giant autolith. These rocks, and the rocks of this area are all in the MZ. Click on the photo to see large autolith locations (red A's), Triple Group locations (red lines), and the macrorhythmic layering section (blue markers).

Looking north toward the prominent macrorhythmic layering section on the lower west flank of Pukugagryggen. When we were on them the Triple Group members were rather hard to distinguish; they are easier to see from far away.

cumulate beds lapping up on the margin of a large anorthositic autolith, near the landing place at the foot of Pukugagryggen. These were deposited by currents that were forced to flow up, over, and around the block as it protruded from the pluton floor.

The end of a very large layered gabbro autolith ~250 m long that extends along the coast of Uttental Sund on the east shore of Kramer Island. Here, a smaller, lighter colored layered gabbroic troctolite autolith is seen. This giant autolith block is not on the McBirney (1989) map. This giant block contains numerous smaller blocks, demonstrating the entrapment of blocks during roof zone crystallization. The light-colored block in the middle of this photo is a gabbroic troctolite block within the giant block (identified as item "1" on Figure 17 and also shown in Figures 18A and 19A of Irvine et al., 1998). This gabbroic troctolite has plagioclase with An74, which is the most primitive autolith and rock in the Skaergaard, and probably represents a piece of UBZT. The giant host gabbro block is equivalent to the MZ, UBZγ, which has no cumulus olivine.

Photo of the southern end of the giant gabbro block, with layered MZ rocks lapping up against the gabbro block sides. The small gabbroic troctolite block in the photo above is also visible here to the right of the man.

Photo of the block interior, with diffuse layering seen below the man's feet and to his right.

Photo of a particularly coarse zone of rock that is visible just above and to the left of the man's feet in the photo above. This coarse rock rests on finer-grained layered rock, also visible in the photo above. Mineralogy includes magnetite, plagioclase, and pyroxene. Thin section here.

Looking northeast along the gabbro block, showing some finer-grained layered rock and the coarser grained rock above it.

Graded layering within the giant gabbro autolith. Note that the layers are normally graded, with mafic bottoms and felsic tops. The layering on the roof therefore grades the same way as layering on the floor. The block is cut by a thin basaltic dike.

An small autolith block caught up in the giant gabbro block. Up is to the left, so the draping of layers around the included felsic autolith is from the bottom. This is the same autolith as that shown in Figure 19D of Irvine et al. (1998).

Numerous small gabbroic troctolite autoliths in the giant gabbro block.

Angular autolith in the giant gabbroic block. Base of the autolith looks like replacement anorthositic gabbro, but the other layers are clear enough. The layering in the host block are at a small angle (~20°) to the layering in the included block. this block also has a very primitive composition and is presumably from UBZT.

Autoliths strewn in a layer in the lower part of the MZ. This suggests that blocks were distributed laterally by a current.

Base of medium size autolith block in LZc, showing irregular form and disturbed graded beds below the block.

Photo of an autolith in LZc, showing some deformation below the block, and thinned layers over the top.

Photo across a gully eroded in a basaltic dike. This shows the sharp upper point of an autolith, with thinned layers draped on the sides and over the block. Another medium size block lies stratigraphically above this one in the middle left.

Graded layer with abundant autoliths strewn within it. Again, this implies that they were distributed by currents.

Photo of a block with slightly deformed layers below, and mafic pegmatite at the block bottom. Pegmatite on block bottoms was fairly common, implying that the blocks were relatively impermeable to upward-percolating liquids.

Impressively dense swarm of blocks enclosed in dark brown MZ mush.

Bottom contact (under the nearest feet) of a gabbroic anorthosite autolith (right) hosted in MZ gabbro (left). The autolith is probably from UBZβ. Diffuse and disturbed layering in underlying rocks to the left may be related to block impact. View is to the east with Uttental Sund in the background.

Photo of a sheath fold immediately below and to one side of the block in the photo above. This is some of the deformation that occurred in the underlying layered series mush at the time of block impact.

Closeup of the interior of the medium size block in this photo sequence, showing that it is very coarse-grained with a distinct plagioclase foliation. The mineralogy is plagioclase-pyroxene with oikocrystic magnetite and interstitial olivine.

The block in the photos above is to the top left, just behind the yellow jacket. This photo shows numerous smaller blocks that lie on top of the larger one. Many of these have olivine- and oxide-rich reaction rinds, with layering lapping up the sides of the blocks.

Closeup view of an irregular block (gray) with a magnetite-rich reaction rind and brown MZ layers lapping up on the block sides.

Photograph of several blocks in the MZ, with clear truncation of a rusty layer in one of the blocks. There are also truncations elsewhere of less obvious layers.

A medium size block, looking east at the down-folded layers beneath the block, with some of the underside deformation visible to the left. Wagers Top is peeking up over the near horizon. This is the same autolith as that shown in Figures 14C and D of Irvine et al. (1998).

Closeup view of the bottom of the block in the photo above, showing deformed layers and thin magnetite replacement veins.

Closeup view of the north end of the block in the photo above, showing squished-out material on the block underside, downwardly deformed layers to the lower right, contorted layers in the lower center, and upward lapping, post-impact layers to the left.

Contorted layer north of the block in the above photos. This contorted layer is some of the poorly consolidated floor material squirted out from under the block at the time of impact, highlighting the impact horizon. The beds below are not very regular, possibly because this area has a great many blocks.

The man is standing on a gabbroic autolith. Stratigraphic tops are to the left, so the rusty layer is interpreted to be an olivine-bearing reaction rind on the bottom of the block. There was quite a bit of argument that the reaction rind might actually be on the top of the underlying block to the lower right, which is mostly obscured by water. Strongly layered area to the right of the man is trough filling at the block margin.

Photo looking at the same area as that above, but from the south and below. The large gabbro autolith is to the upper right, with the magnetite-rich trough layers on the block margin in the center. The olivine-rich reaction rind extends from the upper left to the bottom center, with an offset along the irregular top of the underlying block or blocks. Near woman is standing on the rusty rind, and to her right (your left) is the underlying block, mostly wet, and there is probably yet another block in front of her at the rind offset.

Closeup of onlapping magnetite-rich layering on the south side of another block just south of the blocks in the photos above, in the area labeled "6" in Figure 21 of Irvine et al. (2001b).

View from below of three dikes cutting the UBZT block. The dikes are truncated at the block margins and so must pre-date block foundering. In addition, small anorthositic replacement patches are visible, and a few of these are cut by the dikes and therefore themselves formed in the roof zone prior to dike emplacement.

A close view of one of the pre-detachment dikes crosscutting the block in the photo above. Notice how the dike has irregular layers near the margins and becomes coarse toward its interior.

Lower ends of two of the dikes (here adjacent) at the base of the block in this photo set, where the block and dikes terminate against MZ gabbro. The pencil tip is at the contact between the MZ gabbros below and the dikes and block above.

Mushroom-shaped anorthositic replacement bodies in the block of this photo set. These are viewed from the bottom of the block, so the mushroom caps are up. These replacement bodies are elsewhere cut by the pre-detachment dikes, and so predate them, and so predate block detachment. Notice how the anorthositic material has more mafic margins in their lower parts, but not at the caps.

Photo looking south at the overfolded and disrupted layering on the underside of the medium sized block in this photo set. The block itself is the light colored rock in the right middle. This is essentially the same photo as Figure 12C of Irvine et al. (1998).

View to the east toward Basistoppen. The foreground is the ridge on the southeast margin of Kramer Island, just east of the small lake at an elevation of 17 m. The light-colored rock on the left of the ridge is a giant autolith, with onlapping macrorhythmic layering visible on the right side. This giant block does not seem to be on the McBirney (1989) map, though it shows another very large autolith farther to the north.