Biogenic structures that modify the structure of rocks can be caused by burrowing animals or the penetration of roots in soils. For siltstones and sandstones, wetting the core with a spray bottle can make some of these structures more apparent and help to differentiate them from nodules or other types of disturbed rock fabric, but care should be taken not to wet or repeatedly wet clayey rocks (especially rooted claystones) as this will hasten their breakdown.

Biogenic structures are represented by the third or decimal digit of the Ferm code.

Burrow structures XX8 are features formed by the burrowing action of small creatures such as worms and snails. Vertical burrows are most easily identified in cores, where they form vertical, J-shaped or U-shaped tubes or crack-like features that break across the normal bedding of the rock. Tubes and irregular burrow features are commonly filled by coarser particles. Horizontal burrows look like circular or oval patches of different color or different grain size than the surrounding rock. Many burrows have internal structures that result from a distinctive habit of the creature that formed the burrow. In highly bioturbated fabric, bedding may be destroyed, and individual burrows may be difficult to define because horizontal and vertical burrows cut across each other.

Dark Gray Burrowed Sandy Shale 328
Silt- and sand-filled (light color) burrows in a dark gray shale (mudstone) in core. Small-scale, fine structures can be seen within individual burrows.
Gray Burrowed Sandstone 548
Example of a burrowed sandstone, where all original bedding has been destroyed by the bioturbation.

If the original bedding is the predominant aspect of the sample, but a small number of burrows are present, the rock can be coded to incorporate the burrowing as a decimal digit (XXX.8)

Black Shale with Sandstone Streaks 313
Example of a black shale with sandstone streaks having minor burrow structures (313.8).

Common burrow shapes

Burrows and other types of bioturbation are very common in sedimentary rocks. They form by the activities (dwelling, grazing, moving, etc.) of invertebrate animals, and some of these activities leave distinctive shapes. Although burrows can form in sediments of fresh, brackish, and marine waters, they are most common and variable in shape in rocks deposited under marine or saline waters.

Burrows may be filled with different rock types or grain sizes than the surrounding matrix rock. Burrows and other types of bioturbation may be aligned vertically or horizontally in core. Horizontal burrows tend to appear as circular or oval structures, which may look like nodules. If the core breaks along a horizontal burrow, the butt end of the core will show the oval structures attach to tubular or curving structures in the horizontal plane. Vertical burrows generally appear as thin, straight, tilted, “J”-shaped, or “U”-shaped disruptions through bedding. Some vertical burrows may be upward cone shapes or irregular in appearance. Vertical burrows commonly have a sharp top along bedding.

Intense, closely spaced and overlapping or cross-cutting bioturbation is particularly common in marine siltstones, and can be useful for correlating marine members or beds between coal beds.

Root structures (XX7) are formed by plant roots. The structures are typically subvertical, squiggly or irregular lines that cut through rock bedding or occur in an otherwise massive flint clay (claystone), but they may also be straighter or wedge-shaped. Wedge-shaped and straight roots may easily be confused with burrows, but generally lack complex internal structures, may branch downward, and may be lined with thin films of carbonaceous (black) material. Some roots are filled with silt or ironstone. Pyrite (green and gold patches or halos) is also relatively common around root structures. In very fine-grained rocks, rooting is accompanied by shiny slickensides (polished surfaces) oriented at varied angles. Rocks with root structures are termed fireclay, seat earth, or claystone.

Identifying roots is important

Identifying root structures is important in core descriptions because beds with roots and no overlying coal may be correlative to coals nearby. Also, extensive rooting in fine-grained rocks (especially those with appreciable clay content) within the immediate roof of underground mines can sometimes weaken roof rocks. Identification of potential weak zones in the roof can aid in planning roof support.

Light Gray Green Sandy Fireclay 337
Siderite-filled roots in a light gray, sandy fireclay (337). The vertical oval shapes at the top of the sample could be confused with bioturbation, but the longer squiggly lines are relatively distinctive for roots.
Dark Gray Fireclay 127
Roots with thin coal films in dark shale (127). Note discontinuous squiggly black horizontal and subvertical lines. The mottled appearance and irregular fracture of this core specimen are typical of rooted rocks.

Roots versus burrows

Ancient root structures and burrows can look very similar and be difficult to differentiate, even for experienced core describers. In fact, both can occur in the same layer (common in some ironstone beds formed when coastal wetlands were drowned by the sea). In coal-bearing rocks of the eastern and midwestern United States, roots are easily distinguished when they are lined by a thin film of carbon (coaly), branch downward, or attached to Stigmaria (the fossil root of a coal-swamp tree). Roots in flint clays and claystones are also commonly associated with glassy slickensided surfaces, which are not commonly associated with burrows. Burrows are easily distinguished when they have distinctive J or U shapes in vertical aspect, or sinuous structures on the butt ends of cores. Burrows are also distinctive when they have tiny internal features (called spreite, which look like inclined laminae). Unfortunately, the shapes and sizes of straight to inclined burrows and roots lacking any distinctive features overlap, so are difficult to distinguish. Question marks or notes can sometimes be added to core descriptions to explain any possible uncertainty.

Fossil shell material (XX9) is the remains of seashells (clams, brachiopods), snails, and other creatures in core. Shells are typically white, curved fragments made up of calcium carbonate, which reacts to hydrochloric acid. They can occur in shales, siltstones, sandstones, and limestones, but are particularly common in the lower parts of dark gray to black shale and limestone beds.

Gray Sandstone with Fossil Shells 549
Fossil shells in a gray sandstone (549).




Last Modified on 2019-10-01
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