an interesting sandstone outcrop
an interesting sandstone outcrop
Some geological processes consistently form intriguing shapes that can commonly be mistaken for fossils or other features. For example, dendrites are a branching pattern of manganese oxides that form along fractures in rocks. Superficially, they look rather similar to a branching, fern-like plant, but the pattern is purely a result of diffusion-related chemical processes that can be duplicated experimentally or with computer modelling (it is a type of fractal pattern), and the structures follow along fractures at any orientation, rather than the bedding planes typical for most plant fossils.
The shapes produced by many geological processes are genuinely interesting, and it may take a bit of experience and alot of scrutiny to become convinced that what one is looking at is something non-biological, or that it is a biological structure, but not the one you thought. The shapes can sometimes look quite compelling until examined in detail, and whole chapters of sedimentology texts are sometimes devoted to the most common types of "fossil-like" sedimentary structures, which are usually referred to as "pseudofossils".
This document presents a variety of interesting rock structures, some of which could be considered pseudofossils, depending upon how good your imagination is. Each specimen is presented with a "superficial" interpretation, and then a more detailed one.
This document is still under construction, so all the images are not yet linked in. If you are stumped by any of the currently-unexplained images, drop me an e-mail note for an explanation.
tree rings?
These images show opposite sides of a specimen with concentric colouration
that appears to penetrate the specimen. Could these be tree rings in a
piece of fossil wood?
"cranium" together
"cranium" apartThese images show a rock specimen which has layered pieces of thin material with a curved shape that could represent part of a "skull cap" of a small "human cranium", perhaps from a child. Notice the different coloured layers just in from the surface, perhaps representing different "petrification" for the "marrow" of the skull bone, eventhough the bone is a bit thicker than expected. Alternatively, the material peeled from the surface could represent a "fossil onion". Sample JP96-001.
a piece of an "intestine" or perhaps a "limb bone"
with the articulation surfaces conveniently broken off.Clearly a bilaterally symmetrical cylindrical shape like this could not form by random processes. Sample JP96-002.
a "stomach" or possibly a "liver"The ellipsoidal shape is clearly non-random, and when broken open, this and other specimens contain the remains of fossil ammonites (sticking out of the upper right on this specimen), clams, fish, and fossil lobster, making a "stomach" interpretation more likely (see below).
rounded "gall bladder" shapes with clams
rounded "gall bladder" shapes with ammonite shell
rounded shapes with fish and ammonites
Another possibility is that these represent small stomachs (perhaps from
a child), because the fossil clam and ammonite shells found within
them could represent meals.
a "fossil finger"This specimen bears a striking resemblance to Carl Baugh's "fossil finger" from the Cretaceous of Texas. The great "significance" of this specimen is discussed in this mock press release.
stacked conesWhat are these? They certainly seem too regular to be a natural feature. Maybe they are artificial -- perhaps ice cream cones?
an, uh, interesting shapeThis specimen looks like, uh, well, use your imagination; and don't blame me for what you think up.
a strangely-shaped blobThe arrow points to some sort of structure.
a mineral-encrusted leafThis specimen looks like an oak leaf that has been encrusted with minerals to form a fossil. Several views show the way it had curled up before being mineralized. Arrows point to veins.
an unusual outcropThis pair of specimens was found exposed in an outcrop along a river. Photo by Chris Collom.
leisegang rings
Leisegang rings are concentric bands of alternating coloration, typically
found in sandstones, and typically with an orange, red, or brown colour.
They are usually composed of iron oxides (hence the colour) and form when
oxygen-bearing water diffuses in through a porous rock, often from
fracture surfaces (note the way the concentric structures become
closely parallel to the fracture surfaces at the edges of the specimen).
The water and oxidation processes diffuse inward along a discrete front,
whose position varies depending upon the water and oxygen supply. Deposition
of iron oxides is enhanced at the interface between water and air or
oxygen-bearing and oxygen-depleted water, leaving behind a concentric
zone of iron oxide minerals, not unlike the rings of a bathtub, except
the process occurs in three dimensions. Leisegang rings are typically
a product of near-surface weathering, and can sometimes be confused
with bedding laminations, but their relationship to fractures makes
them fairly obvious (when in situ). In outcrop, concentric
leisegang rings occurred around every fracture in the sandstone cliff this
specimen was obtained from.Cretaceous, from the Rocky Mountain foothills in Alberta.
concretion together
concretion with spheroidally-weathered material apartConcretions are zones of cementation formed in sedimentary rocks. Most commonly, they involve carbonate minerals like limestone (calcium carbonate) or siderite (iron carbonate), and the harder composition causes them to differentially weather out of the outcrop. Concretions can have a wide variety of shapes that are determined by the chemistry of mineral solutions in the sediment, variations in porosity and permeability of the sediment, and often variations in chemistry caused by the decay of animals or plants. The last often causes the formation of concretions around fossils (see below).
In this example, the concretion is siderite-cemented shale. Surrounding shale was soft and dark grey in colour, versus the "rusty" iron oxide coating of this specimen, so the concretions were quite distinctive on the outcrop. Ellipsoidal shapes were most common. As the conretion was weathered, the alteration of the siderite to iron oxides penetrated from the outside towards the interior, mainly as a result of the incremental infiltration of water. This process produced leisegang rings like the previous example. As the alteration continued, the rock expanded slightly due to the increase in volume caused by the hydration of the iron minerals and clays, causing the exterior to spall off in a process known as "spheroidal weathering". The spheroidal weathering fractures preferentially formed along the concentric variations in composition produced by the leisegang rings.
This is what the specimen looked like on the outcrop, prior to collection and reassembly:
outcrop photo
You can see how this is only one of several specimens, and how the
leisegang rings and spheroidal weathering has progressed inwards from
pre-existing fractures in the concretion.
From marine Cretaceous rocks near Jumping Pound Creek, west of Calgary, Alberta.
a concretionThis is one of many calcium carbonate cemented concretions found in tidal marsh sediments at Fort Beauséjour, New Brunswick (near Amherst, Nova Scotia). The sediments are unconsolidated except for the concretions, so the concretions weather out due to wave action and are lying around on the surface of the beach. The concretions have preferentially formed around twigs and pieces of grass or roots, or along variations in the grainsize or composition of the sediment (e.g., siltier), some of which may be controlled by the tubular burrows produced by invertebrates. In addition to plants (mostly Spartina grass), some of the sediments also contain shells of snails and clams that lived in the marsh. The small, arrowed structure is a snail shell that got incorporated into the concretion.
Upright sub-fossil tree stumps, representing an ancient coastal forest that was drowned and buried by marsh sediments, also occur at this locality. There is a good illustration of the sediments of the Fort Beauséjour area at: Fort Beauséjour salt marsh at the Geological Survey of Canada, Terraine Sciences Division collection of Canadian landscapes images.
TO BE CONTINUED...eventually