Where is Iron Concretion in Shale found?
Iron oxide/hydroxide concretion in shale
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Open the appIron Concretion in Shale occurs in specific geological environments. Knowing where to look, and in what kind of rock, is the difference between going home empty-handed and finding a great specimen.
Where to find Iron Concretion in Shale
Ohio, USA
Famous for ironstone concretions, particularly in the Pennsylvanian-age shales of the Appalachian Basin, often containing well-preserved fossils (e.g., Mazon Creek-type concretions).
Illinois, USA
The Francis Creek Shale Member of the Carbondale Formation (Pennsylvanian) is renowned for its ironstone concretions containing exceptional soft-bodied fossils (Mazon Creek biota).
United Kingdom
Common in Carboniferous coal measures (e.g., 'clay ironstone' in shales associated with coal seams) and Jurassic shales (e.g., Cleveland Ironstone Formation).
Germany
Found in various Mesozoic and Cenozoic sedimentary basins, often associated with marine shales.
Canada
Present in shales of various ages, including Cretaceous shales of the Western Canada Sedimentary Basin.
Collecting tips
- Look for Resistant Masses. Concretions are typically harder and more resistant to weathering than the surrounding shale. Look for rounded or irregular lumps protruding from weathered shale outcrops, or as loose, dense masses in stream beds and eroded slopes where the softer shale has been removed.
- Examine Shale Outcrops. Carefully inspect exposed shale layers. Concretions often occur in distinct horizons or layers within the shale. Freshly exposed shale may reveal concretions still partially embedded.
- Check for Fossil Associations. Many iron concretions form around organic nuclei, including fossils. Breaking open concretions (carefully, with appropriate safety gear) can sometimes reveal well-preserved plant or animal fossils within. This is particularly true for septarian concretions.
- Observe Color and Density. Iron concretions will generally be darker (reddish-brown, black) and noticeably denser than the surrounding grey or black shale. A simple 'heft test' can often distinguish them.
Quick Check
- Color:
- Reddish-brown, yellowish-brown, dark brown, black, or greyish-brown.
- Luster:
- Dull to earthy, sometimes sub-metallic.
- Streak:
- Yellowish-brown, reddish-brown, or black (depending on the dominant iron mineral).
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