18 December 2014
One for the ichnologists
Posted by Callan Bentley
For those inclined toward trace fossils…
…This is from Banff National Park, Alberta, Canada. I saw it on the trail to Consolation Lakes from Moraine Lake. I do not recall rock type – could be dolostone, could be Gog quartzite. It’s float (loose; not in situ), but I infer the photographed surface is the underside of the bedding plane; I’d be fine being totally wrong about that, though. There are two things I would like explained/identified here: (1) the prominent, arcuate ridges (which I infer to be trace fossils of some kind) and (2) the finer features, packed in sets, sometimes apparently emanating perpendicularly from the larger arcs, but also present where there are no “arcs.” They look like rodent “gnawing” marks to my untrained eye.
Anyone want to take a stab at identifying these features, and interpreting them for the sake of a poor geoblogger’s education?
Bottom surface, “arcs” are probably horizontal feeding traces – passive infilling of sandstone into a “groove” excavated into underlying fine-grained sedimentary layer, not preserved; “gnawing” marks – paired “toeprints” of a multi-legged creature (trilobite?) being dragged across the underlying sedimentary layer by current.
Hmm, hard to say without rock in hand, but is there any chance that these are not ichnofossils at all? The repetitious arc shapes look ever so much like brach shells in cross section… even a very slight difference in original chemical composition from the host material could provide this level of differential weathering to produce the 3D relief we’re seeing, whether dolostone or quartzite. I’ve never seen horizontal feeding traces where every single trace of significant length was arc shaped, but that seems to be the case here, thus raising my suspicions. The finer features in sets are below my resolution — I have to search an area and find multiple examples of anything so fine before reaching any definitive conclusions. But if not brach shells, I’d lean toward Ron’s horizontal feeding traces as well, but they just doesn’t smell right to me.
I think having two of the traces crossing each other (center-right) is evidence against bivalve cross-sections (and so would the expected age – Proterozoic-Cambrian). they remind me of “Planolites” type trace fossils.
I’m inclined to say those ridges are Thalasanoides (a Shrimp Dwelling Burrow in a firm ground) – that would really only make sense though if it was in the Paleozoic …. The one in the top right bifurcates (y shape), very characteristic of Thalasanoides. I’ve seen some cool modern epoxy examples of the crazy networks these shrimp make that can penetrate down 3 meters from the sediment water interface.
substitute “bivalve and brachiopod cross-sections” in my comment.
am inclined to argue harshly against myself and for you Mike, if indeed those two do cross. with this one sample and one view to look at, my hypothesis looks bankrupt… but looks can be deceiving and I’ll cling to its possible viability a bit longer. fun post, Callan.
This is Gog Group, lower Cambrian quartzite. Magwood in his MSc has the ‘tube trace fossils’ as _Planolites tubularis_’ — I am not going to say he is right or wrong for I do not deal with the tubular trace fossils that often.
The ‘finer features’ I feel that I can comment on: if tightly grouped with limited medial line then I would state that it is _Cruziana_, a wider distance between the right and left ridge sets would be _Diplichnites_ and the random ridges I would refer to as ‘Indeterminate Arthropod Scratches’. These would have been made as the arthropod[-like] organism (yes, quite possibly a trilobite) walked/ran/dug into the sand which was blanketing a mud (most likely an organically-rich mud). One day I will have the ‘arthropod-like’ traces from this region published!