Discover the Alportian Substage and Explore the Fossils and Geodiversity That Shaped Our Planet
- Wayne Munday
- 4 days ago
- 3 min read
Sip back and discover the Alportian Substage dating back to between 323 - 322 million years ago and explore the fourth subdivision of the Namurian Stage within the regional Carboniferous Period of the British Isles and northwest Europe. Positioned between the Chokierian and Kinderscoutian substages it is defined by a principal biostratigraphic zone of Homoceras undulatum a species of goniatite a type of extinct cephalopod belonging to the family Homoceratidae. Although lacking a formally ratified Global Boundary Stratotype Section and Point (GSSP) the Alportian is well established regionally through characteristic fossil assemblages notably goniatites, microspores and foraminifera. First described in 1933 by William Gunn from stidies of exposures in Alport Dale, Derbyshire, the substage is distinguished by the progradation or sediment accumulating seaward forming the Pennine Delta. These shales are linked to shale gas reservoirs and include marine bands formed during episodes of elevated sea levels. The Alportian Substage helps to interpret the geological evolution of the Pennine and Craven Basins as well as the Pennine Delta system.
The Alportian Substage is a clearly defined interval of a million years within the Namurian Stage of the Carboniferous Period and ranks as the fourth of seven substages in the Namurian laying between the Chokierian and Kinderscoutian substages.
Geologists rely on the Alportian to piece together the sequence of geological events during the Carboniferous Period especially in Britain and northwest Europe. Although the Alportian does not have a formal GSSP it is recognised internationally and is associated with the Mid-Carboniferous Boundary a transitional interval between the lower and upper parts of the Carboniferous and reflects shifts in marine fauna and especially goniatites.
The Alportian Substage in Britain is recorded through a variety of formations that reflect contrasting depositional environments. In northern England, the fossil-rich Edale Shales and the Bowland Shales are Carboniferous in age and comprise dark grey to black, organic-rich shales, frequently containing marine fossils and representing potential hydrocarbon sources. The Edale Shales are laterally continuous with the Bowland Shales, indicating deposition within the same temporal and environmental framework. The Samlesbury Formation of the Millstone Grit Group contains both Chokierian and Alportian rock including the delta-derived Brocka Bank Grit.
In the Pennine and Gainsborough Basins thick “Alport Sandstone” show substantial deltaic progradation whereas deeper basinal settings such as the Widmerpool Basin feature condensed hemipelagic mudstones of the upper Bowland Shale Formation deposited in low-energy environments with mixed washed, blown and biogenic input. Collectively, these successions illustrate the dynamic interplay of marine sedimentation, deltaic advance and basinal condensation during the Alportian Substage. These contrasting facies tell a story of the development of the Pennine Delta system.
The Pennine Delta system developed during the Carboniferous Period when the region was submerged beneath warm, shallow seas and dominated by extensive deltaic sedimentation. Within the fault-controlled block-and-basin topography of the Pennine Basin, contrasting depositional environments emerged between subsiding basins and uplifted blocks. River deltas prograded into these basins depositing thick successions of sandstones, mudstones, and siltstones notably the Millstone Grit. Early stages of basin development also supported shallow marine carbonate deposition producing significant limestone units. As sedimentation continued vast coal swamps developed across low-lying areas giving rise to the coal measures that remain a defining feature of the Pennine landscape. Collectively, these successions record the dynamic interplay of tectonics, sea-level change, and sediment supply that shaped the Carboniferous geology of northern England.