Your Backyard Geology – Part 9

Concretions are not shaped by running water or fabricated by humans, nor are they dinosaur eggs. They are fascinating geological formations that come in the most interesting shapes. Concretions are time capsules holding clues into the past geologic record.

Concretions are widespread in sedimentary rocks worldwide, including the Yukon. They have also been found on Mars. The first concretions formed hundreds of millions of years ago. They formed in almost every geological time period since and are still being formed today. They can have an irregular shape, but most often they look like cannon balls, perfectly rounded, or like the kind of saucer-shaped UFOs you’d see in a 1950s sci-fi movie.

They range in size from microscopic to several metres across. Concretions are harder than the surrounding rock they formed in. This allows them to erode intact out of the softer rock. In some locations, many of these eroded concretions will be scattered all over the ground. Quite often they are on beaches where waves have pounded them out of the surrounding rocks.

They inspire place names such as: Valle de la Luna, Cannonball River, Kettle Point, Bowling Ball Beach, Pumpkin Patch and Rock City. Here in the Yukon, there is Thunderegg Creek, slightly misnamed as they are not true thundereggs. (Real thundereggs are formed from volcanic rock and have different minerals inside, but they can look like concretions from the outside.)

Concretions form mainly in shale, sandstone and siltstone. A chemical reaction takes place to cement localized portions of the host sediment. Concretions form best in sediments that are in the very early stages of hardening into rock. The clay cliffs in Whitehorse are a good example of this. The silt and sand has been deposited, but it will be millions of years yet before they harden into rock.

The concretion starts with a seed or nucleus within the sediment. Quite often it is something organic like a shell fragment, twig, leaf, fish bones or animal feces. When groundwater reaches these organics, they start to breakdown. They release elements and compounds such as ammonia, calcium and carbon. This changes the acidity (pH) of the groundwater around the nucleus. If the groundwater is high in calcium, the change in pH causes the mineral calcite to precipitate out as a solid around the nucleus.

If conditions are right and there is a good supply of calcium in the groundwater, the process continues. Calcite starts depositing out in all directions in concentric rings and the concretions grow. Observations have shown they can grow as fast as one to 10 centimetres outwards every year.

If the sediments are very consistent in porosity and composition, the concretion grows outward in a perfect sphere. If there are layers in the surrounding sediment that are less porous, such as a clay seam, the concretion will tend to grow flattened. It will spread in a horizontal direction and become oval or disc-shaped. If the sediment is very inconsistent, concretions can grow in irregular shapes.

Calcite is the most common cement in concretions. Cement can also be iron oxide, silica (quartz), pyrite and other minerals. It all depends on the local conditions. Calcite in groundwater could come from a nearby limestone, or from shells within the host sediment. Iron would come from the breakdown of iron-bearing minerals.

No matter what the cement, the process is the same. A good example of how this process works is illustrated in the Four Corners area of the southwest United States. The Navajo Sandstone in that area is rich in iron oxide and the mineral hematite. The word hematite comes from the Greek for ‘blood-like’ because of its red colour.

In some areas the red sandstone has been “bleached” white by groundwater flowing through it, collecting the iron and moving it away. The host sandstone can take on a beautiful red and white striped appearance. The concretions formed from the iron-rich water are a bright red colour.

In 2004, spherical blueberry-sized objects were found by the NASA Mars exploration rover, Opportunity. These turned out to be hematite concretions. These ‘blueberries’ in the Endurance and Eagle Craters are very similar in size and composition to ones found in the Navajo Sandstone in Utah.

Some concretions have cracks running through their interior. These are called ‘septaria.’ These can form exotic patterns and be filled with different minerals that add to the uniqueness and beauty of each formation. The cracks are thought to occur when the inside of some concretions first start to dry.

Not only are concretions the most amazing shapes, they contain valuable information about the time they formed. The cement preserves everything and freezes the rock and its contents in time. They can tell scientists the direction of ancient groundwater flow and pollen trapped in concretions can give the age and environment in which the sediments were deposited.

The nucleus itself can be the most interesting part of a concretion. Fossil shells or imprints of the original organism can be preserved. In salt marshes along the English Channel, iron-carbonate concretions have been growing around shrapnel and unexploded bombs from World War II. Some have grown to a size of 40 centimetres since the end of the war.

Along the Porcupine River, about 15 kilometres downstream of Old Crow, is a layer of concretions in a clay/silt bluff. In 1997, an oval-shaped concretion about 40 by 70 centimetres in size was recovered from the area. It was already naturally split down the middle. When it was opened up, there was a perfectly preserved impression of a whitefish, 40 centimetres long.

Scientists from the Geological Survey of Canada, the Canadian Museum of Nature and several universities studied the concretion. They determined the fish was somewhere between one and two million years old. The preserved detail on the fossil was good enough to establish it as the oldest known whitefish of that genus on the planet.

Pollen inside the concretion identified the types of trees and shrubs that were around at the time. Researchers were able to determine the fish lived in a lake in a slightly cooler climate than at present.

Throughout the world, concretions have connections to Indigenous cultures. In the ancestral villages of the Four Corners Indigenous people, many iron oxide concretions have been found. They were used as cooking stones. They were also used ceremonially. Red ochre was made from the hematite and used as a pigment for dyeing and rock paintings.

The Maori people of New Zealand, near the famous Moeraki Boulders on the south island, believe that the boulders on the beach were eel baskets that washed ashore when a legendary canoe sunk. The concretions there are spherical and measure up to three metres in diameter.

The Chippewas of the Kettle and Stony Point First Nation on Lake Huron have made it a priority in the preservation of their culture to preserve concretions on their territory.

Concretions can be found in many places in the Yukon. Places like Thunderegg Creek west of Haines Junction, the Watson River and in shale around kilometre 210 of the Dempster Highway. Elongate concretions up to five metres long have been found in the Richardson and Ogilvie Mountains. They can be in any sedimentary rock you come across including shale, siltstone, sandstone and limestone. Where groundwater conditions are right the many “clay” or silt cliffs around the Yukon are good areas to look.

If you want to see some fascinating pictures of concretions from around the world, check out: