Overview 
The Belemnite Marls (averaging 23m), are a series of bluish-gray marls that are a lot paler than the Black Ven Marls that they rest upon. This is due to the Belemnite Marls having a much higher carbonate content (they are said to be kerogen rich). 

Originally called the Belemnite Beds and the Stonebarrow Beds.

At the base of this division is the Apoderoceras Bed (Bed 105 - sometimes called the Armatus Limestone Bed). This is a 0.35m thick limestone bed that forms a reef at Westhay Water, called Hawkfish Ledge, that is revealed at low tide. 

Just down from the top of the division is Bed 120, the Pyritic Marl and Crumbly Bed. This bed forms a prominent lighter band along the base of Wear Cliffs between Seatown and Golden Cap. 

At the top of this division is the Belemnite Stone (Bed 121), which is a 0.15m thick laterally persisting limestone, that forms a reef at St. Gabriels, that is revealed at low tide. 

Although formed during a time when the sea level was fairly deep, the Apoderoceras Bed and the Belemnite Stone bed were formed during a time when the sea had shallowed quite significantly. The Apoderoceras Bed also underwent erosion leading to non-sequence of deposition. 

Throughout the Belemnite Marls there is a distinct colour variation between beds of lighter marl (containing a higher carbonate content), and the darker marls (containing a higher organic carbon content). It is thought that this sort of colour alternation was formed as a result of the 19,000-23,000 year long precession cycle of the earth's orbit. Calcium carbonate measurements through the Belemnite Marls (lower Pliensbachian) are characterised by two scales of cyclicity that can be firmly linked to orbital precession (20 ka) and the 123 ka component of eccentricity. 

According to the book titled 'Cyclostratigraphy and the Early Jurassic timescale' and authored by G. P. Weedon and H. C. Jenkyns:

"This formation was deposited during early Pliensbachian time, at a paleolatitude of about 35°N, in an epicontinental sea that was largely enclosed by the supercontinent of Pangea. The sequence contains compositionally diverse light and dark marl bedding couplets, the thickness' of which are notably reduced in the upper third of the section. The regularity of the couplets in segments of the sequence, combined with a regular amplitude modulation, indicates an origin that is related to the orbital-precession cycle (i.e., one of the Milankovitch parameters). A timescale is developed by assigning a duration of 20,000 per couplet, and this suggests that the entire (Belemnite Marl) formation represents at least 1.78 million years"

Further understanding of this can be obtained by study of the Milankovitch Theory.

Fossils 

This division is famous for Belemnite fossils, including examples of Passaloteuthis and Hastites. Ammonite fossils are also common in this division and include the pyritised, compressed and keeled Tropidoceras, as well as Apoderoceras, Platypleuroceras, Beaniceras, Liparoceras, Lytoceras, Tragophylloceras, Tragophylloceras ibex, Uptonia jamesoni and the large Parainoceramus ventricosus. Also found is the gastropod Amberleya.

Ammonite Tragophylloceras
Ammonite Tragophylloceras

 Zones and sub-zones 

Zone

Subzone

Beds

Tragophylloceras ibex

Beaniceras centaurus 

121

Acanthopleuroceras valdani

118d-120

Uptonia jamesoni

Tropidoceras masseanum

118c

Uptonia bronni

115-118b

Platypleuroceras brevispina

110-115

Phricodoceras taylori

105-109

Beds and fossils 

Bed Bed name Geology Info
121 Belemnite Stone
[see image below]		 A laterally persistent limestone
120 Pyritic Marl & Crumbly Bed
[see image below]		 Friable & pyritic marls with impersistant lenticules of crinoidal limestone Upper Marls
119 Belemnite Shales
[see image below]		 Shale containing fossils Chlamys rollei and Tropiorhynchia thalia
118d Upper Darker Marl Darker marl with fossils Chlamys rollei and Tropiorhynchia thalia
118c
118b
118a
117 Darker marl 
116 Upper Dark Band Middle Marls
115 Upper Pale Band
[see image below]
114 Middle Dark Band
113 Middle Pale Band
[see image below]
112 Lower Dark Band Containing ammonite Cincta sp
111 Lower Pale Band
[see image below]		 Containing ammonite Cincta sp
110b Lower Darker Marl Darker marl  Lower Marls
110a Darker marl 
109 Lower Paler Marl
[see image below]		 Paler marl
108 Paler marl
107 Paler marl
106 Paler marl
105 Apoderoceras Bed
(Armatus Limestone Bed)
[see image below]		 16" laterally persistent limestone

 

Bed photographs

Middle Marls

This photo shows the relative positions of the Middle Marls on the cliff face when seen exposed at Westhay Water, (the water from which causes the darkening of the cliff face). These marls a distinctive pattern of three parallel light coloured bands throughout the Belemnite Marls exposure of Black Ven and Stonebarrow Hill. The upper pale band is Bed 115, the middle pale band is Bed 113 and the lower pale band is Bed 111. The top of the cliff is known as the  Hawkfish Ledges, and is formed by the Three Tiers formation.

Lower paler marls

A piece of the Belemnite Marl taken from within Beds 106-109.

Apoderoceras Bed

A piece of the Apoderoceras Bed 105. This bed marks the top of the Black Ven Marl division. The small white lenses and calcite crystal inclusions within the rock.

Surface of the Belemnite Marl

This is the surface of the foreshore below the Wear Cliffs, midway between Golden Cap and Seatown. The photo shows the exposed surface of Bed 119, some 2m below Bed 120. Erosion by seawater between high tide and low tide has resulted in several genera of Belemnite fossils being exposed. Tidal action of sand and shingle has marked the surface of the fossils. The marl is fairly hard making it difficult to extract the fossils without breaking them.

bed-120.jpg (95958 bytes)

This photo shows a lens of crinoid limestone that has been exposed by tidal action. The Crumbly & Pyritic Marls Bed (Bed 120) above the lens has been eroded away. The surface is uneven and is dotted with Belemnite rostra, pieces of fossilised Oyster shell and Crinoid stems and frond pieces.

bed-120-cu.jpg (91341 bytes)

This photo is a close-up of part of the above lens of crinoid limestone. Both the Belemnite and Oyster shell fossils can easily be seen.

bm-04.jpg (37632 bytes)

This photo is of a piece of crinoid limestone found loose on the foreshore under Stonebarrow Hill. You can see on the top surface of the lens the thicker stems and the thinner fronds of Crinoids. Much of the sharp relief and details has been lost due to tidal action.

bm-03.jpg (24905 bytes)

A close-up of another piece of a lens of crinoid limestone from the bottom of the Crumbly & Pyritic Marls Bed (Bed 120) found loose on the foreshore under Stonebarrow Hill. With less erosion of the surface, the cross section of the lens shows the easily identifiable star shape remains of fossilised Crinoid stems.

Belemnite Stone

This is a photo of a piece of Belemnite Stone (Bed 121). You can easily see the circular cross sections of the Belemnite fossils contained within the stone. The photo is in the correct bedding plane; the bottom edge of the rock is the bottom edge of the Belemnite Stone bed.

Belemnite Stone

This photo is of the same piece of Belemnite Stone, however, this photo shows the top surface of the bed. Several Belemnite fossils can be seen laying along the surface and dipping into the bed.

 Other Belemnite Marl references