Introduction
Column showing the succession of Lower Jurassic strata to be found on the coast and underlying country south of the River Tees.
Following the double inundation of the Late Triassic Rhaetic episode, the resulting noxious waters gradually ameliorated as the sea deepened and the area became a shallow arm of the Tethys Sea. Lower Jurassic rocks were deposited in a variety of marine environments ranging from shallow to deep sea floor. Collectively the Lower Jurassic succession is referred to as the Lias Group.
The earliest Jurassic beds in the Tees Valley crop out as scars on the foreshore at both Coatham and Redcar. Between here and Saltburn the rocks gradually disappear and the sandy beach is backed by thick deposits of much younger boulder clay. The scar at Saltburn marks reappearance of the solid rock and, in the cliffs, we see the second phase of deposition, the Staithes Formation, in the form of shallow marine sandstone and mudstone.
At Huntcliff, ironstone seams crop out at near the cliff top, some 90m above sea level, and a disused fan-house associated with Huntcliff Ironstone Mine can be seen on the cliff top. These seams mark the third phase of deposition in very shallow water. Rocks deposited in the final phase of Lower Jurassic sedimentation across the Tees Valley occur near to Hummersea and Boulby but also inland, for example along the escarpment of Guisborough Forest and at Slapewath. These rocks demonstrate an initial deepening of the Tethys, which later underwent an episode of low oxygen-content, before shallowing, culminating in the sea-floor being lifted above sea level, with the exposed surface suffering erosion and weathering to produce an episode of non-deposition, or unconformity
Redcar Mudstone Formation
Curving scars of Redcar Mudstone extend seaward near the lifeboat slipway at Redcar.
After the noxious conditions of the Triassic marine incursions, the sea deepened, became fully oxygenated, and a new era of sedimentation commenced with the Redcar Mudstone Formation. The Formation can be further sub-divided into Calcareous Shales, Siliceous Shales, Pyritous Shales and Ironstone Shales. The Redcar Mudstone is perhaps best known for its extinct fossil oysters or Devil’s Toe-Nails which wash up on the beaches in great numbers. At their base near Redcar these rocks comprise cycles of soft mudstone capped by thin, hard limestone often packed with well-preserved fossils. The Formation is well-exposed in coastal sections between Coatham and Staithes, or at Robin Hood’s Bay but seldom crops out inland due to a thick covering of till.
Staithes Formation
Staithes Formation at Cowbar Nab.
During deposition of the preceding Redcar Mudstone, waters became ever shallower and closer to a landmass causing a change in the type of sediment laid down on the sea floor. The Staithes Formation comprises fine-grained sandstones, thin mudstones, and bands of iron-rich nodules. Finely laminated sandstones, up to 0.8 metres thick, are frequently followed by units in which the bedding has been almost completely obliterated by burrowing organisms, a process know as bioturbation. Episodic storm surges resulted in ultimate deposition of finely-laminated beds known as striped beds or tempestites. The Staithes Formation is possibly the most fossiliferous rock to be found locally, with extensive oyster beds, belemnites, trace fossils left by many creatures that burrowed within the sediments, and much more. Sedimentary structures such as ancient ripple marks are also commonly seen within these rocks.
Cleveland Ironstone Formation
Cleveland Ironstone Formation at Jet Wyke east of Staithes.
This suite of rocks are famous for the part they played in the growth of Teesside during the mid- to late-1800s. Beds belonging to the Cleveland Ironstone Formation were deposited in conditions of varying water depth. They are made up of layers of grey silty mudstones with five distinct seams of ironstone, the latter deposited during episodes of low sediment input, in a sea rich with life. Fossils are common and usually well-preserved in the ironstones. The seams, in order of deposition, have been dubbed Avicula, Raisdale,Two-foot, Pecten and Main Seams. They increase in both thickness and grade upwards culminating in the Main Seam which possesses an iron content of 33% and maximum thickness of 4.8 metres in the Eston Hills.
Cleveland ironstone often demonstrates an oolitic texture, the rock comprising a high percentage of structures around a millimetre in diameter known as ooids. These are concentrically ringed, sub-spherical, particles of berthierine, an iron silicate mineral and siderite (iron carbonate). Ooids form through the action of wave energy rolling around small particles of sand, shell, or some other suitable nucleating matrial, upon which are precipitated minerals from the surrounding water. Because of their mode of formation the presence of ooids generally indicates deposition in shallow water. Cleveland ironstone is blue-grey, or grey-green in colour when fresh and somewhat mottled when oolitic, weathering to a rusty red-brown.
Whitby Mudstone Formation
Grey mudrock of the Whitby Mudstone Formation (Alum Shale Member) as seen in Loftus Quarry.
Following deposition of the Cleveland Ironstone the sea reached depths not experienced across the area since Permian times. This led to deposition of the Whitby Mudstone Formation which consists of, in ascending order, the Grey Shale, Mulgrave Shale, Alum Shale and Cement Shale Members. A few younger rock units, in the shape of the Peak Mudstone, Rosedale Oolite and Foxcliff Siltstone Members, are not present except in a few pockets of the North York Moors having either not been deposited or removed by erosion. Initially, sea level change was gradual and there is little difference between the Grey Shale Member of the Whitby Mudstone Formation and those of the preceding Cleveland Ironstone Formation.
Succeeding the Grey Shale is the Mulgrave Shale Member which comprises, in ascending order, the much quoted informal units Jet Rock, Top Jet Dogger, Bituminous Shales and the Ovatum Band. A double band of pyrite-skinned concretions often containing the rare ammonite Ovaticeras.
The Jet Rock consists of beds of dark finely-laminated shale containing pyrite. This shale smells strongly of mineral oil when freshly broken. Seasonal falls of dead plankton from the upper waters are responsible for the presence of the oil. Seams of jet can be found which form where waterlogged tree trunks became buried within the oxygen-depleted sea floor mud.
The Top Jet Dogger is a discontinuous thin limestone best viewed in coastal exposures. This is succeeded by the Bituminous Shales which have similar characteristics to the Jet Rock. Shallower seas led to greater oxygenation of the waters and deposition of beds known as the Alum Shale Member, informally divided, in ascending order, into the Hard, Main Alum and Cement Shales. Many large reptile fossils (Plesiosaurs, Ichthyosaurs, crocodiles, and even a Pterosaur) have been recovered from these beds some of which are on display in local museums.
In the North York Moors area the Alum Shale Member is usually succeeded by the Cement Shales. These beds differ little in appearance from the Alum Shales, but contain numerous large limestone nodules once processed for hydraulic (Roman) cement.
« Triassic Middle Jurassic »
©2011 Tees Valley RIGS Group.
Lower Jurassic
Introduction
Column showing the succession of Lower Jurassic strata to be found on the coast and underlying country south of the River Tees.
Following the double inundation of the Late Triassic Rhaetic episode, the resulting noxious waters gradually ameliorated as the sea deepened and the area became a shallow arm of the Tethys Sea. Lower Jurassic rocks were deposited in a variety of marine environments ranging from shallow to deep sea floor. Collectively the Lower Jurassic succession is referred to as the Lias Group.
The earliest Jurassic beds in the Tees Valley crop out as scars on the foreshore at both Coatham and Redcar. Between here and Saltburn the rocks gradually disappear and the sandy beach is backed by thick deposits of much younger boulder clay. The scar at Saltburn marks reappearance of the solid rock and, in the cliffs, we see the second phase of deposition, the Staithes Formation, in the form of shallow marine sandstone and mudstone.
At Huntcliff, ironstone seams crop out at near the cliff top, some 90m above sea level, and a disused fan-house associated with Huntcliff Ironstone Mine can be seen on the cliff top. These seams mark the third phase of deposition in very shallow water. Rocks deposited in the final phase of Lower Jurassic sedimentation across the Tees Valley occur near to Hummersea and Boulby but also inland, for example along the escarpment of Guisborough Forest and at Slapewath. These rocks demonstrate an initial deepening of the Tethys, which later underwent an episode of low oxygen-content, before shallowing, culminating in the sea-floor being lifted above sea level, with the exposed surface suffering erosion and weathering to produce an episode of non-deposition, or unconformity
Redcar Mudstone Formation
Curving scars of Redcar Mudstone extend seaward near the lifeboat slipway at Redcar.
After the noxious conditions of the Triassic marine incursions, the sea deepened, became fully oxygenated, and a new era of sedimentation commenced with the Redcar Mudstone Formation. The Formation can be further sub-divided into Calcareous Shales, Siliceous Shales, Pyritous Shales and Ironstone Shales. The Redcar Mudstone is perhaps best known for its extinct fossil oysters or Devil’s Toe-Nails which wash up on the beaches in great numbers. At their base near Redcar these rocks comprise cycles of soft mudstone capped by thin, hard limestone often packed with well-preserved fossils. The Formation is well-exposed in coastal sections between Coatham and Staithes, or at Robin Hood’s Bay but seldom crops out inland due to a thick covering of till.
Staithes Formation
Staithes Formation at Cowbar Nab.
During deposition of the preceding Redcar Mudstone, waters became ever shallower and closer to a landmass causing a change in the type of sediment laid down on the sea floor. The Staithes Formation comprises fine-grained sandstones, thin mudstones, and bands of iron-rich nodules. Finely laminated sandstones, up to 0.8 metres thick, are frequently followed by units in which the bedding has been almost completely obliterated by burrowing organisms, a process know as bioturbation. Episodic storm surges resulted in ultimate deposition of finely-laminated beds known as striped beds or tempestites. The Staithes Formation is possibly the most fossiliferous rock to be found locally, with extensive oyster beds, belemnites, trace fossils left by many creatures that burrowed within the sediments, and much more. Sedimentary structures such as ancient ripple marks are also commonly seen within these rocks.
Cleveland Ironstone Formation
Cleveland Ironstone Formation at Jet Wyke east of Staithes.
This suite of rocks are famous for the part they played in the growth of Teesside during the mid- to late-1800s. Beds belonging to the Cleveland Ironstone Formation were deposited in conditions of varying water depth. They are made up of layers of grey silty mudstones with five distinct seams of ironstone, the latter deposited during episodes of low sediment input, in a sea rich with life. Fossils are common and usually well-preserved in the ironstones. The seams, in order of deposition, have been dubbed Avicula, Raisdale,Two-foot, Pecten and Main Seams. They increase in both thickness and grade upwards culminating in the Main Seam which possesses an iron content of 33% and maximum thickness of 4.8 metres in the Eston Hills.
Cleveland ironstone often demonstrates an oolitic texture, the rock comprising a high percentage of structures around a millimetre in diameter known as ooids. These are concentrically ringed, sub-spherical, particles of berthierine, an iron silicate mineral and siderite (iron carbonate). Ooids form through the action of wave energy rolling around small particles of sand, shell, or some other suitable nucleating matrial, upon which are precipitated minerals from the surrounding water. Because of their mode of formation the presence of ooids generally indicates deposition in shallow water. Cleveland ironstone is blue-grey, or grey-green in colour when fresh and somewhat mottled when oolitic, weathering to a rusty red-brown.
Whitby Mudstone Formation
Grey mudrock of the Whitby Mudstone Formation (Alum Shale Member) as seen in Loftus Quarry.
Following deposition of the Cleveland Ironstone the sea reached depths not experienced across the area since Permian times. This led to deposition of the Whitby Mudstone Formation which consists of, in ascending order, the Grey Shale, Mulgrave Shale, Alum Shale and Cement Shale Members. A few younger rock units, in the shape of the Peak Mudstone, Rosedale Oolite and Foxcliff Siltstone Members, are not present except in a few pockets of the North York Moors having either not been deposited or removed by erosion. Initially, sea level change was gradual and there is little difference between the Grey Shale Member of the Whitby Mudstone Formation and those of the preceding Cleveland Ironstone Formation.
Succeeding the Grey Shale is the Mulgrave Shale Member which comprises, in ascending order, the much quoted informal units Jet Rock, Top Jet Dogger, Bituminous Shales and the Ovatum Band. A double band of pyrite-skinned concretions often containing the rare ammonite Ovaticeras.
The Jet Rock consists of beds of dark finely-laminated shale containing pyrite. This shale smells strongly of mineral oil when freshly broken. Seasonal falls of dead plankton from the upper waters are responsible for the presence of the oil. Seams of jet can be found which form where waterlogged tree trunks became buried within the oxygen-depleted sea floor mud.
The Top Jet Dogger is a discontinuous thin limestone best viewed in coastal exposures. This is succeeded by the Bituminous Shales which have similar characteristics to the Jet Rock. Shallower seas led to greater oxygenation of the waters and deposition of beds known as the Alum Shale Member, informally divided, in ascending order, into the Hard, Main Alum and Cement Shales. Many large reptile fossils (Plesiosaurs, Ichthyosaurs, crocodiles, and even a Pterosaur) have been recovered from these beds some of which are on display in local museums.
In the North York Moors area the Alum Shale Member is usually succeeded by the Cement Shales. These beds differ little in appearance from the Alum Shales, but contain numerous large limestone nodules once processed for hydraulic (Roman) cement.
« Triassic Middle Jurassic »
©2011 Tees Valley RIGS Group.