Belemnite Marl

Overview

Originally called the Belemnite Beds and the Stonebarrow Beds.

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).

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. 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.

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 Water, that is revealed at low tide.

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 can be obtained by study of the Milankovitch Theory.

Exposures

Belemnite Marl as found between Lyme Regis and Charmouth:

Belemnite Marl as found between Charmouth and St. Gabriel's Mouth:

Belemnite Marl as found between St. Gabriel's Mouth and Seatown:

Fossils

This division is famous for Belemnite fossils, including examples of Passaloteuthis and Hastites.

A 112mm long Passaloteuthis apiculata found in the
talus at the base of the Golden Cap exposure of the Green
Ammonite Bed.

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.

A Tragophylloceras ibex ammonite,
sourced from Herve Chatelier.

Also found is the gastropod Amberleya.

Zones, sub-zones and fossils

Zone	Sub-zone	Fossils
Tragophylloceras ibex	Beaniceras centaurus	121
Tragophylloceras ibex	Acanthopleuroceras valdani	118d-120
Uptonia jamesoni	Tropidoceras masseanum	118c
	Uptonia bronni	115-118b
	Platypleuroceras brevispina	110-115
	Phricodoceras taylori	105-109

Beds, geology and fossils

Bed	Name	Geology	Grouping
121	Belemnite Stone	A laterally persistent limestone
121
120	Pyritic Marl & Crumbly Bed	Friable & pyritic marls with impersistant lenticules of crinoidal limestone	Upper Marls
120
119	Belemnite Shales	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
114	Middle Dark Band
113	Middle Pale Band
112	Lower Dark Band	Containing ammonite Cincta sp
111	Lower Pale Band	Containing ammonite Cincta sp
110b	Lower Darker Marl	Darker marl	Lower Marls
110a	Lower Darker Marl	Darker marl
109	Lower Paler Marl	Paler marl
108		Paler marl
107		Paler marl
106		Paler marl
106
105	Apoderoceras Bed (Armatus Limestone Bed)	16" laterally persistent limestone
105

Photographs

Cross section of Belemnite Stone [Bed 121].

At Westhay Water, the Middle Marls of Beds 111 to 116 are seen.
This 'marker' can be seen throughout all Belemnite Marl exposures.

Although the Belemnite Marl is fairly hard, cracks due
to erosion often appear in the cliff face.

Then without warning, it collapses!

A 'lenticule' of the Pyritic Marl and Crumbly Marl Bed.

A fault line extending down from the Green Ammonite Bed into the Belemnite Marl,
provides a weak point that is attacked by the sea. The softer Belemnite Marl is
soon eroded away.

Taken a year later, this photo shows that the Belemnite Marl of the beach exposure,

The middle marl bands of the Belemnite Marl are easily seen on this exposure under Stonebarrow Hill. [Oct 2004]

Belemnite Marl	Black Ven Marl	Down Cliff Sands	Eype Clay Bed
Green Ammonite Bed	Junction Bed	Shales with Beef	Thorncombe Sands	Three Tiers Bed