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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.