DESCRIPTION:

The dark brown to reddish brown Marlstone Rock Formation crops out in North Oxfordshire around Banbury. It's an iron-rich (ferruginous), fossil-rich ooidal (composed dominantly of ooids - ooids are small spheroidal,  sedimentary grains, usually composed of calcium carbonate), limestone. It can be interbedded with cemented (calcareous) sandstone and ferruginous mudstones. In some locations it may be so iron-rich that can be described as ooidal ironstone.  It contains abundant marine fossils.  The Marlstone is usually well stratified and well sorted, and usually cross-bedded.

 

The iron content (as ooids, altered shell material or in the groundmass) is berthierine (dark green iron-rich mineral formed in low-oxygen marine conditions), altering to siderite (iron carbonate). 

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There are two distinct facies (a sedimentary facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment):

 

1. berthierinic/sideritic oolith-shell fragmented limestones

2. thinner more sideritic, sandy limestones.

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The former facies provides the economically valuable iron ore.

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LOWER BOUNDARY:

The base of the formation is determined at the downward change to mudstones and siltstones of the Dyrham Formation. The transition is usually erosive and conglomeratic band up to 0.3 metres thick, which consists of phosphatic or ferruginous tabular pebbles of limestone or mudstone. In some places the formation may rest on a sandstone bed at the top of the Dyrham Formation, which it may be impractical to map separately from the Marlstone Rock.

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UPPER BOUNDARY:

The boundary has been established at the point where there is a transition to the mudstone/nodular limestones of the Whitby Mudstone Formation.

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THICKNESS:

The formation is heavily cambered and for this reason thickness estimates can be unreliable.  Isopachyte maps put the maximum thickness between 7.6 m - 10 m (at Bloxham).

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DEPOSITIONAL ENVIRONMENT:

In the Middle Lias there was a general lowering of sea level, resulting in a progressive regression. The sedimentation of the Marlstone Rock Formation therefore most likely occurred in a series of relatively shallow, interconnected basins between the London Platform and the Severn Basin. This would account for the variations in thickness.

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The faunal species are predominately sessile (organisms that are fixed in one place; immobile eg barnacles) sea floor dwellers.  Burrowing species are absent, quite probably because the ooidal sands would have shifted regularly in proximal zones (close to shore) and more turbulent storm phases. The faunal assemblage is dominated by typically of nested of rhynconellid, Tetrarhynchia tetrahedra and Lobothyris punctata brachiopods. Several species of the ammonite Pleuroceras, and numerous belemnites and bivalves, rare corals, crinoids and plant material are also present. The zone index fossil Pleuroceras spinatum is rare, but has been found at Redlands Quarry, Hook Norton and at Fawler. Belemnites occur abundantly in the conglomeratic bed at the base of the formation, but are rare later in this deposit.

 

GEOGRAPHIC DISTRIBUTION:

The Marlstone Rock Formation outcrops widely across northern Oxfordshire and east of the Cherwell Valley.

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LANDSCAPE CONTRIBUTION:

The formation, which is more resistant to erosion than the over-lying Upper Lias strata, characteristically forms a shelf or bench along the lip of the scarp face and  along valley sides. I t forms an extensive plateaux between  Deddington, Adderbury, Wroxton and Hook Norton.  The Sor Brook and its tributaries, drains the marlstone plateaux in a south-easterly direction towards the Cherwell, cutting deep, down through the Middle Lias units to the Charmouth Mudstone Formation.

 

BIOSTRATIGRAPHY:

The formation is present from within the Amaltheus margaritatus to the end of the Dactylioceras tenuicostatum ammonite zones.

 

ECONOMIC IMPORTANCE:

Known as locally as Hornton Stone or Banbury Ironstone.

 

Building stone (ashlar and rubble).  Much of the stone that has been quarried, was used for purposes other than architectural. Many of the villages of the Banbury, Wroxton and Deddington district owe their special character to its rich gingerbread hues. In Banbury Samuel Pepys Cockerill's church of St Mary (1797) is resplendent Hornton ashlar. In Oxford, Deane & Woodward used the stone  for string courses and ornamental work round the windows of the University Museum (1859) and Christ Church Meadow buildings (1862-65). Butterfield also used Hornton Stone together with Bristol sandstone in a chequer-effect in the chapel at Balliol College.

 

From the late 19th through the first half of the 20th centuries it was predominantly a source of iron ore. The exploitation of the Marlstone Rock Formation for iron ores began in the 1850's, with pits opened at Fawler in the west of the area and sites along the Cherwell Valley eg. Steeple Aston and Adderbury to the east. These sites were identified as viable because of the proximity of local rail lines.  Later the Oxfordshire Ironstone Railway and the King's Sutton to Cheltenham GWR line gave rise to the industrial scale quarries at Hook Norton, Bloxham, Wroxton and Alkerton .