Sample EC96-001 -- interpretation

NOTE: Some of these images are rather large. Check the sizes before downloading. I could decrease the size, but only at the expense of important details. If you want smaller images anyway, let me know.

Weathered exterior view, reflected light

These images were photographed with a macro lens.

Fig.1 (152Kbytes) Fig.2 (25Kbytes) Fig.3 (46Kbytes) External view of specimen, with "narrow end" (right -- Fig.1), lateral view (left -- Fig.2), and "wide end" (Fig.3). Note the saw marks on the ends. These were present on the specimen when it was received. The specimen was also already coated with clear laquer, which was partially removed with acetone. The specimen is about 12cm long, and 4cm ("narrow end") to 5cm ("wide end") in diameter. Figs.1-4 are NOT at the same scale.

ec96-001 lateral view Fig.4 -- lateral view (39Kbytes) showing outer surface, with specimen flipped over from orientation in Fig.2. Note small black lichens on the surface to the right of centre.

Interpretation: The exterior of this specimen has a rough, irregular texture at several millimetre to centimetre scale (Fig.2 and 4). No indications of tubular, haversian canal-like structure were observed on the exterior. In cross section, the specimen has two distinct zones of material -- an outer zone of well-cemented material, and an inner, soft core of clayey, unconsolidated material. This has been partially excavated out of one end (the "wide end", in Fig.3). Both zones have a similar light yellow-green colour, although the inner part of the outer zone is slightly greyer, and the "wide end" is slightly red-brown, and there are some variations to grey, orange, or dark brown colours. In cross section, the central zone is variable in shape (see below), but ranges from 1 to 2cm in diameter.

Polished cross sections, reflected light

To investigate the internal structure of specimen EC96-001, it was cut with a rock saw into three segments, designated EC96-001A, EC96-001B, and EC96-001C. EC96-001A is about 1cm thick, and is from the "narrow end" (left in Fig.2 and 4), EC96-001B is also about 1cm thick, and is the next portion of the specimen, and EC96-001C is the remainder (the "wide end", to the right in Fig.2 and 4), comprising about 3/4 of the original specimen. Only EC96-001A is illustrated below, but the others look much the same. Each cut surface was polished by hand to show surface features. Because the central core of the specimens was quite fragile, it was partially plucked out during the preparation process. Note that some thickness was permanently lost due to the width of the saw blade and the polishing.

The images were photographed using a reflected light microscope.

EC96-001A -- first segment from "narrow end"

Fig. 5A -- cross section A, same side as "wide end" (296Kbytes), polished surface. Digitally reconstructed from two images. Low magnification.

Fig. 5B -- cross section A, same side as "wide end" -- annotated version of Fig.5A (76Kbytes)

Interpretation: As can be seen in this image, the central core of the specimen is not always circular in cross section. Based upon 5 cross section surfaces (the two sides of EC96-001A, EC96-001B, and one side of EC96-001C), it appears to vary from rounded to more lenticular and irregular shapes depending upon position along the length of the specimen, and these variations correspond to bulges on the exterior of the specimen. Several features are visible on this surface, keyed by number to illustration 5B:

  1. Dark brown, high lustre (metallic), cubic cross-section structure. This is a partially altered cubic crystal of the mineral pyrite (FeS2), also known as "fools gold". The pyrite is partially weathered to a dark brown colour, probably an iron oxide or hydroxide mineral (?goethite). This type of alteration is common for pyrite.
  2. At the opposite end of the central zone is a cube-shaped cavity, probably representing the former location of another pyrite grain that has been completely weathered out due to chemical alteration.
  3. Around the pyrite grains are dark brown zones of mineralization permeating the spaces between the grains in the surrounding rock (see description of 5 below). These appear to be more iron oxides (goethite?) related to the alteration of the pyrite.
  4. An inner zone of unconsolidated material is found in all cross sections. It consists predominantly of yellow to orange soft clay surrounding grey translucent or clear transparent quartz grains and other unidentified mineral grains. The boundary between the core and surrounding material is gradational over less than a millimetre. The yellowish colour is likely caused by the present of the iron "oxide" mineral limonite (H2Fe2O4(H20)x), a common product of the chemical weathering of iron-bearing minerals.
  5. Around the central core is well-consolidated material consisting predominantly of dark grey quartz grains. Between the hard quartz grains are soft zones of more yellow-orange clay and limonite identical to that of the central core. Additional minerals may also be present, but are unresolvable at this scale.
  6. Further out, in diffuse contact over 2-3 mm, is a lighter yellow, slightly less cemented zone, also consisting of grey quartz grains and softer yellow clay, but with a slightly higher proportion of yellow clay minerals.
Fig. 6 -- cross section A, same side as "wide end" enlargement of lower left of Fig.5 (160Kbytes). Moderate magnification.

Fig. 7 -- cross section A, same side as "wide end" enlargement of upper left of Fig.5 (160Kbytes). High magnification.

Fig. 8 -- cross section A, same side as "wide end" enlargement of lower left of Fig.5 (208Kbytes). Same magnification at Fig.7. High magnification.

Fig. 9 -- cross section A, same side as "wide end" enlargement of upper right of Fig.5 (224Kbytes). High magnification.

Interpretation: At this level of magnification, individual quartz grains (grey, resistant) are visible, as is the plucking (depressions) of the softer clays and limonite. Fig.8 shows the diffuse boundary between the greater cementation in the inner versus outer parts of the outer, cemented zone. The diffuse zone of iron oxide mineral (goethite?) around the pyrite grains is also visible.

Fig. 10 -- cross section A, same side as "narrow end" (152Kbytes). This is almost the same level as the original "narrow end" of the specimen (i.e. Fig.1), but it has been ground down a few millimetres and polished. Note the original saw marks on the right. White square marks the location of Fig.11. Low magnification.

Interpretation: Much the same as Fig.5, but note the more circular shape of the cross section, and the absence of visible pyrite grains (although the dark blotch near the boundary of the inner unconsolidated zone and outer cemented zone on the right side may be related to a grain further into the specimen).

Fig. 11 -- cross section A, same side as "narrow end", enlarged (160Kbytes). High magnification.

Interpretation: This close-up of the inner unconsolidated zone shows the lumpy texture of the clays and limonite, and occasional isolated grey or clear/colourless quartz grains (e.g., just below the centre of the image). There are no indications of tubular haversian canal structures.

Comparison

Comparison of specimen EC96-001 with dinosaur bone microstructure. All illustrations are polished surfaces in reflected light. Each pair of images is at the same magnification.

Fig.12 -- dinosaur bone cross section (left) and EC96-001A (right -- part of Fig.5) (120Kbytes). Same scale. Scale in millimetres in left image. Low magnification.

Fig.13 -- dinosaur bone cross section (left) and EC96-001A (right -- part of Fig.8) (248Kbytes). Same scale. Scale in millimetres. Moderate magnification.

Interpretation: There is little to interpret from this comparison. The structure is completely different. There is no indication in any of the cross sections or the exterior surface of specimen EC96-001 of bone microstructure in the form of tubular haversian canals. The internal structure is inconsistent with fossil bone. The varying, sometimes irregular shape of the inner zone is also inconsistent with its interpretation either as marrow or bone itself.

The specimen consists of a tubular structure of variably-cemented fine-grained sandstone. The inner zone is completely unconsolidated, while the outer zone is cemented by silica (quartz) and possibly other minerals. The specimen appears to have been intensively chemically weathered, as indicated by the alteration of pyrite crystals and the presence of limonite. It is likely the current state does not reflect the original mineralogy, but judging by the proportion of quartz to non-quartz grains, this was a fairly impure, probably lithic sandstone.

There are several possibilities to explain the tubular shape:

  1. rhizolith -- a zone of cementation formed around a plant root in a fossil soil horizon
  2. burrow infill -- a tubular burrow infilled with sand
  3. plant root or trunk -- infilled by sand
  4. a concretion, with shape determined by variations in porosity, fluid flow, or composition in the sediment
There is no indication of the leaf or rootlet scars expected for a plant fossil (3), and without more information (e.g., the geometry of the specimen in the field, or the presence of branching), this possibility and a burrow interpretation (2) can not be eliminated. A rhizolith (1) is an appealing possibility because of the significant difference in cementation between the inner and outer zones. A cemented, tubular shape is characteristic of rhizoliths, but again, this possibility can not be conclusively demonstrated or excluded without more field information. Finding these structures in a vertical orientation in the bedding of a paleosol (which do occur in the Llewellyn Formation) would make a rhizolith interpretation much more likely. The possibility this specimen represents a non-biologic structure like a concretion (4) can not be excluded either. Since this specimen was not collected in situ from bedrock, field information is likely to remain unavailable unless similar specimens are recognized in place. The degree of alteration of the specimen also makes identification more difficult.

Although thin section and SEM examination could resolve the petrology of this specimen further, it seems pointless given the obvious inconsistency between its structure and bone in magnified polished surfaces observed in reflected light.


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Andrew MacRae macrae@geo.ucalgary.ca