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    Dungy Head

    The Dorset Coast

    England


A.    The Purbeckian Dorset


 

DORSET ABSTRACTS


Mammal and Cinder 4th Order Sequences:

    Stratigraphic columns interpreting the cyclic hierarchy from six localities on the Dorset coast (including Durlston Bay, Worbarrow Tout, Mupe Bay, Lulworth Cove (east), Stair Hole and Durdle Door) are presented below.  4th and 5th order cycles are indicated by arrows at the right of the columns and PACs (6th order cycles) are shown by tick marks.  Clements' and Ensom's bed numbers are shown to the left of the Durlston and Worbarrow localities respectively.  Cycles at each locality are documented with field photographs.  The Dorset Coast.jpg


Column: Durlston Bay Mammal 4th (ppt)

    Photographs documenting Mammal 4th column at Durlston Bay.  In photo 1. the meter-scale rests on a gray soil with ped structure.  The mammal bed (mb on the column) is the dark brown soil at 60 cm on the scale. 1. DB mammal bed.jpg    Photo 2.  shows 5th order sequences B and C.  The top of the meter-scale is at the boundary surface between these sequences.  The top of sequence B is a dark gray soil with in place root structures.   2. DB mammal-FWC.jpg    Photo(s) 3.  show root structure in the dark soil at the top of sequence B in the Mammal 4th and an approximate modern analogue (mangroves in mud flats) Cat Island the Bahamas.  3. Root and mangrove swamp.jpg

  

Column: Durlston Bay Cinder 4th (ppt)

    Photographs documenting Cinder 4th column at Durlston Bay (DB).  In photo 4. the top of the meter-scale is at the boundary surface between 5th order sequences A and B.  Sequence B is the Cinder member (~4 m in thickness).  4. DB Cinder 4th.jpg    Photo 5. shows the top of the Cinder member (Sequence B) and limestone-shale packages of sequences C and D and the 4th order boundary at the top of the Cinder 4th.  5. DB Cinder 4th-2.jpg

 


Column: Worbarrow Mammal + Cinder 4ths (ppt)

    Photographs showing an overview of the section and a close-up of the upper part of the Cinder 4th at Worbarrow Tout.  Photo 6. is a distant view showing dark terrigenous rocks of the Kimmeridge Shale at the base of the cliff, the massive limestone of the Portland Group in the vertical cliff and the base of the Purbeck Group in the upper third of the cliff face.  6. Worbarrow KPP.jpg   Photo 7. show the top of the Cinder 4th and the overlying Intermarine 4th at Worbarrow Tout.  The C and D 5th order sequences of the Cinder 4th are condensed into single limestone-shale couplets (under the feet of the geologists).  7. Worbarrow Cinder.jpg

Column: Mupe Mammal + Cinder 4ths (ppt)

    Photographs of the condensed Mammal and Cinder 4th order sequences at Mupe Bay (MB).  Photo 8. shows the unconformable boundary at the base of the Mammal 4th at the lower red arrow (labeled 3rd/4th) and the boundary between the Mammal and Cinder 4ths (upper arrow).  The A  5th order sequence is missing in the Mammal 4th.  The correlative position of the Mammal bed is in the unconformity at the third order boundary.  8. MB Mammal 4th.jpg   Photo 9. contains the Cinder 4th (yellow arrows mark the boundaries).  Just above the lower boundary there is a shale (soil) covered by slumping in the picture representing all that remains of the A - 5th order sequence.  The entire 4th order sequence is more condensed than the correlative section at Worbarrow. 9. MB Cinder 4th.jpg
     

Column: Lulworth Mammal Cinder 4th (ppt)

    Photographs of the condensed Mammal and Cinder 4th order sequences at Lulworth Cove East (LC).  Photo 10. shows the unconformable boundary at the base of the Mammal 4th at the lower red arrow (labeled 3rd/4th) and the boundary between the B and C 5th order sequences (upper yellow arrow).  The A and D 5th order sequences are missing in the Mammal 4th.  The correlative position of the Mammal bed is in the unconformity at the third order boundary.  10. LC Mammal 4th.jpg    Photo 11. shows the Mammal and Cinder 4ths at Lulworth Cove.  The Mammal 4th between red arrows and the Cinder 4th above the upper red arrow.  The B/C 5th order boundary indicated by a yellow line.  11. LC Mammal/Cinder 4ths.jpg

 
Column: Stair Hole Mammal Cinder 4th (ppt)

    Photographs of the condensed Mammal and Cinder 4th order sequences at Stair Hole (SH).  Photo 12. shows the unconformable boundary at the base of the Mammal 4th at the green arrow on the right (labeled 3rd/4th), the boundary between the B and C  5th order sequences (yellow arrow) and the upper 4th order boundary with the cinder 4th order sequence (left green arrow).  The A and D  5th order sequences are missing in the Mammal 4th.  The correlative position of the Mammal bed is in the unconformity at the third order boundary.  S12. SH Mammal 4th.jpg   Photo 13.  depicts the boundaries of the highly condensed Cinder and Intermarine 4th order sequences.  Red arrows mark boundaries between 4th order (400 ka) sequences.  The arrow at the right marks the boundary at the base of the Cinder 4th. 13. SH Cinder/Intermarine 4ths.jpg 


Column: Second Third Order Sequence Stair Hole 

    Four  condensed 4th order sequences are preserved in the  3rd order (2 ma) sequence below the mammal bed (one 4th order is missing in unconformity).  Photo 14. depicts four 5th order (100 ka sequences) each condensed to a single limestone shale couplet in the 4th order sequence that occurs just below the mammal bed.   14. SH Top 4th in 3rdjpg    Photo 15. shows the third fourth order sequence in this third order.  15. SH 4th Order three.jpg


Column: Durdle Door Mammal Cinder 4th (ppt)  
    Photo 16. shows the preserved parts of the 3rd order sequence below the Mammal Bed at Durdle Door.    16. DD Section up to Cinder 4th.jpg


Correlated columns:  
    The Mammal 4th order sequence (yellow) shown in relation to the Cinder B 5th order sequence (green) from Durlston Bay in the east (right) to Durdle Door to the west (left).  Note marked decrease in thickness but continuity of the Cinder and Freshwater chert beds. 
Correlation Cinder Cherty 4ths   Compare with individual columns showing cyclic structure above.


A THIRD ORDER SEQUENCE at Durlston Bay, Dorset, Mammal Bed to the Broken Shell Limestone (with F.W. Anderson faunicycles) 

    The Third Order Sequence consisting of five 4th order sequences is shown with stratigraphic columns for Durlston Bay beginning with the lower most Mammal 4th.  Interpretation of cyclic structure is shown to the right of the field log and formations, members, Clements' bed numbers and F.W. Anderson faunicycles are shown to the left of the log.  Most faunicycles are correlative with 5th order (100 ka) sequences. Exceptions occur in more restricted parts of the stratigraphic interval where single faunicycles may correspond to two or even three 5th order sequences. 

DB Mammal 4th-I  FC

DB Cinder 4th-II  FC

DB Intermarine 4th-III  FC

DB Scallop 4th-IV  FC

DB Corbula 4th-V FC

    The first two fourth order sequences in the next 3rd order sequence are shown in the next column correlated with ostracod faunicycles. 
DB Broken Shell Ls 4ths I+II  FC


 

DORSET ABSTRACTS

 


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