Early Jurassic (Sinemurian – Toarcian) palynology of the Lias, Lyme Regis, Dorset

Clarissa Wright

Project aims

• Correlate palynomorph occurrences and abundance trends with established biostratigraphic schemes.

- Morbey (1978) pollen & spores forNW Europe- Powell (1992) dinoflagellate cystsfor NW Europe- Wall (1965) acritarchs in Lyme Regis, Dorset

• Deduce palaeoenvironmentalimplications based onpalynomorph and palynofacies analysis.• Based on outcrop sample materialranging from the Sinemurian to theToarcian.

Sample no. Location Lithostratigraphic fm. Member Bio Success. Age Ammonite Zones

1 - 4 The Cobb E Sinemurian

3 Blue Lias Blue Lias Semicostatum

5 – 7b Stonebarrow Cliff E Sinemurian

L Sinemurian

5 Charmouth Mudstone Black Ven Marl Turneri

7a Charmouth Mudstone Black Ven Marl Obtusum

8 - 23 Golden Cap E Pliensbachian

L Pliensbachian

8, 10 Charmouth Mudstone Belemnite Marl Jamesoni/ibex

13 Charmouth Mudstone ?Belemnite Marl Jamesoni/ibex

14 Charmouth Mudstone Green Ammonite Margaritatus

16, 18 Dyrham fm Three Tiers “

22 Dyrham fm Eype Clay “

24 - 37 Eype Mouth L Pliensbachian

E Toarcian

24 Dyrham fm Eype Clay “

27 Dyrham fm Down Cliff Sand “

30, 32 Dyrham fm Thorncombe Sand “

35 Beacon Limestone fm Levesquei –

spinatum

36 Bridport Sand fm Down Cliff Clay Levesquei

38, 39 Burton Bradstock L Toarcian

38, 39 Bridport Sand fm Bridport Sand Levesquei

Lias Group in the UK

SIN

EMU

RIA

N

E

L

PLI

ENSB

AC

HIA

N

E

L

TOA

RC

IAN

E

L

Material & Methods

Material had been previously prepared using standard preparation techniques (following CGG standards in 1983).

Optical light, x1000 mag., laterally scanning of slides.

Palynomorph analysis: Count of 250 palynomorphs (pollen, spores, acritarchs, dinoflagellates) on oxidised material.Palynofacies analysis: Count of 500 particles of organic matter –AOM, palynomorphs, phytoclasts on unoxidised material.

Limitation: Poor sample preservation

Scale bar = 10μm.

Biostratigraphy – pollen & spores

• Pollen and spore assemblages compared to scheme of Morbey 1978 for NW Europe.

SIN

EMU

RIA

N

E

L

PLI

ENSB

AC

HIA

N

E

L

TOA

RC

IAN

E

L

Ch

ron

ost

rati

grap

hy

Biostratigraphy - Dinoflagellates

Where dinoflagellates are identified, they match the scheme of Powell (1992).

Nannoceratopsis gracilis zone

Luehndea spinosa zone

Liasidium variabile zone

Dapcodinium priscum zone

X

X

Biostratigraphy - Acritarchs

• Species occurrences compared to Wall (1965), in a study of the Lias from Lyme Regis.

• Results show consistency, aside from one case.

• Micrhystridium wattonensis is identified earlier than claimed by the Author.

• Peaked abundance in L Toarcian.

PalynofaciesToarcian• Phytoclast

dominated• Sparse

assemblages

Pliensbachian• Palynomorph

and phytoclast dominated

• Influx of freshwater algae

Sinemurian• AOM dominated• Sparse

assemblages

0 100 200 300 400 500

3i

5

7Ai

24

26

27

28

30

32

35

36

38

39

Sam

ple

no

.

Relative proportions of Phytoclasts, Palynomorphs and AOM.

PHYT

PLYN

AOM

SIN

EM.

E

L

PLI

ENSB

AC

HIA

N

L

L

TOA

RC

IAN

E

L

Trends broadly correspond to sample lithologiesand stratigraphic episodes in the Wessex basin discussed by Hessselbo (2008).

Climate implicationsClassopollis pollen & spores

• Classopollis vs. Bisaccate ratio

• Spore vs. Bisaccateratio

Likewise utilised by (Pienkowski & Waksmundzka, 2006) in Poland

Bisaccate pollen

• Potential implications for climate and proximity to shore.

• Consistently abundant, except in Obtusum and Levesquei zone.

Temperature implications agree with deducations by Van de Schootbruggeet al (2005), and Dera et al 2012.

Conclusion

• Pollen, spores, dinoflagellates and acritarch trends show correlation with established schemes and publications.

• Palynofacies analysis implies environmental conditions of offshore/anoxic in the Sinemurian, basinal-shelfal in the Pliensbachian, and shelfal in the Toarcian.

• Thermophilic indicators may reflect warming in the Sinemurian, and cooling in the Late Pliensbachian.

Acknowledgements: Thank you to Jim Fenton, Guy Harrington and Ian Boomer, and teaching associates of this course.