Septarian Concretion: What Is It and Characteristics

Septarian concretion (also called a Septarian nodule or simply Septarian) is a unique type of rock formation characterized by its striking cracked or geometric pattern. These formations are composed of a combination of minerals, such as calcite, aragonite, barite, and sometimes pyrite, encased in a hard outer layer of limestone or mudstone.

A concretion is a hard, compact mass of matter formed by the precipitation of mineral cement within the spaces between particles, and is found in sedimentary rock or soil.

Septarian from Muddy Creek, Orderville, Kane Co., Utah. Credit: Will Hough

 

What Is Septarian Concretion

A septarian concretion is a type of concretion where the interior exhibits cracks or fissures (septaria) that have been filled with minerals, typically calcite, which precipitated from groundwater. These formations occur in sedimentary rocks, where an initial mass (often around an organic nucleus) hardens, then shrinks or undergoes stress, leading to cracks that later get filled with minerals, creating a distinctive pattern when the concretion is cut open or broken apart.

The term “septarian” comes from the Latin word “septum,” meaning partition, referring to the internal fractures or separations in the rock.

Cracks are highly variable in shape and volume, as well as the degree of shrinkage they indicate. Although it has commonly been assumed that concretions grew incrementally from the inside outwards, the fact that radially oriented cracks taper towards the margins of septarian concretions is taken as evidence that in these cases the periphery was stiffer while the inside was softer, presumably due to a gradient in the amount of cement precipitated.

There is an important distinction to draw between concretions and nodules. Concretions are formed from mineral precipitation around some kind of nucleus while a nodule is a replacement body.

Characteristics of Septarian Concretions

Structure and Shape

• Concretion: A concretion is a mass of sediment or mineral that hardens around a nucleus, such as a fossil, shell fragment, or organic material.
• Fractured Patterns: Septarian concretions are notable for their polygonal cracks or veins inside, often forming a honeycomb-like or mosaic structure.
• The outer shell is usually smoother and harder, made of limestone or mudstone.

Mineral Composition

Septarian concretions are typically composed of clay, calcium carbonate (calcite), aragonite, and other minerals. The cracks or cavities within the concretion are often filled with minerals such as calcite, barite, pyrite, or even chalcedony.

• Calcite (CaCO₃): Fills the cracks or veins, creating yellowish or brownish areas.
• Aragonite: A less stable form of calcium carbonate, also present in some septarians.
• Barite: A secondary mineral that sometimes forms in the veins.
• Limestone/Mudstone: Forms the outer shell.

Color

• Yellow, brown, beige, and gray tones are common due to calcite veins and the outer limestone.
• Some concretions have dark gray or black veins, especially when organic materials are present in the cracks.

Septarian Concretions from the Late Cretaceous Cenomanian Stage, Found West of Orderville, Utah

 

Formation of Septarian Concretion

Septarian concretions are found within sedimentary rocks like mudstone and sandstone. The exact process leading to the distinctive septaria, or internal cracks, within these concretions remains a topic of geological debate.

Formation Process:

• Sediment Deposition: These structures typically originate in tranquil marine environments. Here, an organic nucleus, like a deceased marine creature or shell, gets swiftly enveloped by sediment, setting the stage for concretion formation.
• Concretion Development: Over millennia, minerals present in the water, commonly calcium carbonate or silica, begin to precipitate around this organic core. This process cements the surrounding sediment, gradually forming a hardened nodule or concretion.
• Cracking: Subsequent to their formation, these nodules undergo physical changes. As they lose moisture or experience shifts in internal or external pressure, they begin to develop a network of cracks. This cracking is pivotal for the unique characteristics of septarian concretions.
• Vein Filling: Following the formation of cracks, mineral-laden groundwater seeps into these newly created spaces. Here, minerals like calcite crystallize, filling the cracks and often creating a visually striking contrast with the darker body of the concretion. Other minerals might also precipitate, depending on the local water chemistry, adding to the concretion’s complexity.

Septaria usually contain crystals, often calcite, that precipitated from circulating solutions. Siderite or pyrite coatings are also occasionally observed on the wall of the cavities present in the septaria, giving rise respectively to a panoply of bright reddish and golden colors.

Where to Find Septarian Concretions

Septarian concretions can be found all over the world, but they are most common in sedimentary rocks that were deposited in marine environments. Some of the best-known localities for septarian concretions include:

• The Moeraki Boulders in New Zealand
• The Isle of Wight in England
• The Jurassic Coast in England
• The Smoky Hills region of Kansas, USA
• The Mancos Shale region of Utah and Colorado, USA
• The Bearpaw Shale region of Montana and Saskatchewan, Canada

Calcite inside Septarian concretion. Photo: DweadPiwateWoberts

 

Famous Septarian Concretions

A spectacular example of septarian concretions, which are as much as 3 meters (9.8 feet) in diameter, are the Moeraki Boulders. These concretions are found eroding out of Paleocene mudstone of the Moeraki Formation exposed along the coast near Moeraki, South Island, New Zealand. They are composed of calcite-cemented mud with septarian veins of calcite and rare late-stage quartz and ferrous dolomite.

Moeraki Boulders.

Very similar concretions, which are as much as 3 meters (9.8 feet) in diameter and called "Koutu Boulders", litter the beach between Koutu and Kauwhare points along the south shore of the Hokianga Harbour of Hokianga, North Island, New Zealand. The much smaller septarian concretions found in the Kimmeridge Clay exposed in cliffs along the Wessex Coast of England are more typical examples of septarian concretions.