Calciclastic mass flows from the Darriwilian/Sandbian boundary at Abbereiddy Bay, Pembrokeshire, Wales, UK: BASIN DYNAMICS RESEARCH GROUP

Implications for Avalonian sea-level correlation. BDRG Mitten, A. J.*; Montenari, M. and Pringle J.K. *[email protected], Basin Dynamics Research Group, Wiliam Smith Building Keele University, Keele, Staffordshire, ST5 5BG, UK.

Introduction and Geological Setting

The succession of Abereiddy Bay, St. David's Head, Pembokeshire has previously the subsequently overlying D. murchisoni Shale (c. 2-43m); the top of 456 only been studied for its paleontological significance. The area is the type locality which corresponds to the Abereiddian/Llandeilian boundary. The Dicranograptus of the Abereiddian of the lower Darriwilian (Ordovician). The Castell Limestone boundary is also marked by the erosional deposition of the Castell Shale Upper Sandbian

Formation is situated upon the Darriwilian/Sandbian boundary and consists of Limestone Formation and the subsequent Dicranograptus Shale (c. gracilis 15m of mixed limestone and mudstone units. This formation does not possess a 180m). The deposition of the Dicranograptus Shale continues through Aurelucian Castell palaeoenvironmental interpretation, due to its lack of exposure and the majority of into the Sandbian. 458 Limestone published literature focussing upon the complex fossil assemblages of the The Caerhys Shale Formation was deposited coevally with the underlying Caerhys Shale Formation. Didiymograptus Hendre Shale in Carmarthen; these where described by Williams et al. murchisoni The Castell Limestone Formation is situated on the northern side of Abereiddy (2003) as 'anoxic graptolite bearing mudstones'. Moving towards the Ash and Shale Llandeilian Orodovician Bay. The bay itself is characterised by the east/west trending Llanrian Syncline south (younging), the Caerhys Shale Formation is erosionally overlain teretiusculus that's hinge lies within the centre of the bay. The northern limb of this fold is by the Castell Limestone Formation (c. 12m). Cyffredin Shale Member overturned; this is easily recognisable from the bedding/cleavage relationship Figure 1 (right) – The geological map of Abbereiddy Bay (modified from Rushton et Middle Darriwilian shown in the mudstone units of the formation. murchisoni Llanvirn al. 2000), along with the GVS (ages from Tucker et al., 1990; Tucker and McKerrow Volcanics 1995; Rushton et al., 2000; GVS modified from Waltham 1971) for the area providing

The Didymograputs bifidus Shale (c. 180m) forms the base of the Abereiddian Abereiddian a legend for the geological map. Note the small schematic outline of England and Didymograptus

which is divided by the Llanvirn Volcanics (c. 90m) (Fig. 1). The D.bifidus Shale is Wales indicating the location of Pembrokeshire, and the subsequent map of artus bifidus Shale

conformably overlain by the Didymograptus murchisoni (D. murchisoni) Ash and Pembrokeshire noting the location of Abereiddy Bay. 466

Data and Results Methodology

Total Spectral gamma ray and sedimentological data was collected from (ppm) K (%) U (ppm) Th (ppm) Th/K U(Aut.) 1080 END

1070 the Castell Limestone Formation. The data was collected in at the 1060 N S same time and from the same position to aid with correlation. The 1050

1040 6 Facies Lithology Sedimentary Other Interpretation Facies 14m sedimentological data was recorded as a sedimentary log recording 1030 Code and Structures Features No. 1020 Texture lithological and facies descriptions. The spectral gamma ray data was 1010 M.M. Fine well Massive Nodular Low energy 1 1000 collected using a RS-230 BGO Super Spec hand-held gamma ray 2 sorted black concretion background 990 spectrometer. The RS-230 recorded 120s assay readings every 10cm 980 mudstone. deposition. 970 Pl.M. Fine well Planar Nodular Low energy 2 6 within the Castell Limestone Formation. Within the lower Caerhys 960 sorted black lamination, concretion background 950 mudstone. some deposition. Shale Formation the spectral gamma was taken as 90s assay 940 13m asymmetrical 930 readings every 20cm, this was due to the generically high count 2 ripples. 920 conditions of the shales and the study mainly being conducted on the 910 M.Slt. Well sorted Massive One Higher energy 3 fine grey to instance of background 900 6 Castell Limestone Formation; any Caerhys Shale counts where 890 black normal sedimentation. 880 siltstone. grading. recorded so that an idea of background sedimentation may be 870 6 N S M.Lst. Well sorted Massive Some Distal 4 860 pure 100% dewatering deposition but recorded. 850 clean micrite structures still above the 840 12m grey to black present, an CCD. 830 3 limestone. instance of 820 4 810 5 cracking, 800 6 nodular Palaeoenvironment 790 concretions. 780 C.Slt. Well sorted Massive, More proximal 5 18 770 4 fine grey to planar background 760 black lamination. sedimentation 750 siltstone, above the CCD, 740 11m 5 730 with with a higher

720 calcareous clastic input.

710 material, 700 much like a 690 N calcarenite. 5 S 680 Bre. Well sorted Massive Normal Upper flow 6 670 black grading regime deposit. 660 C 12 650 mudstone Th(ppm) 640 3 matrix, very 10m 630 coarse sand 620 to boulder 610 sized dark 600 4 limestone 590 Caerhys Shale Formation

580 Lower Castell Mudstone Bedset 570 Table 1 - Facies scheme developed for the Castell Lower Castell Limestone Bedset 560 1 Middle Castell Limestone Unit 550 B Limestone Formation at Abbereiddy Bay. The facies 3 Upper Castell Limestone Unit 540 6 9m code in the left of the table shows facies codes, these 530 1 2 3 4 6 520 correlate to the numbers annotating the sedimentary K(%) 510 log in figure 2. 500 3 S Figure 4 - Th/K radioelement scatter plot showing facies units, see figure 2 for 490 480 lithostratigraphical subdivision. 470 A

460 450 Lower Limestone Bedset and Carbonate factory 440 8m Upper Castell Unit Silty shelfal material 430 Rip-up 420 clasts

410 Middle Castell Unit

400 2 Hybrid Beds

390 380 Distal calciturbidite 370 lobe 360 2 350 B Majority of Middle 340 N Bedding 7m Castell Unit 330 plane 320 C.Slt. 310 N S 300 Pl.M.

290 M.Slt. 6 280 Concretionary growth 270 Bre.

260

250 4 6m 240 Lower Castell 230 C Mudstone Bedset 220 6 210 Brecciated deposition 200 Channelised calciturbidite deposit 190 M.M. 2 180 A M.Lst. 170 6 Caerhys Shales Formation 160

150

5m 140 130 6 120 F i g u r e 2 - S p e c t r a l 110 Figure 5 - A paleoenvironmental reconstruction proposed for the Castell Limestone g a m m a r a y a n d 100 Figure 3 - Facies photoplates (left) and interpreted and Caerhys Shale Formations. It suggests that the Castell Limestone Formation was 90 sedimentological data schematic (right). A shows the Bre. facies of the M.F. deposited as several distinct species of sub-aqeous mass flows that all originated from 80 collected of the Castell 70 facies association. C shows the finer lower energy a hinterland carbonate factory. The schematic image shows that the Lower Castell 60 Limestone Formation and 2 facies and associations. B shows the C.Slt. and D. Mud. Limestone Bedset was deposited as a proximal calciclastic debrite, along with the 50 t h e C a e r h y s S h a l e 4m 40 Facies associations and their gradational relationship Upper Castell Limestone Unit. The Middle Castell Limestone Unit was deposited as 30 Formation. The gamma 1 leading up to the next event bed downcutting in the right medial hybrid event beds (as seen within the thicker event beds of the lithological log). 20 ray data displayed shows 10 of the image. C shows the limestone and mudstone of The majority of this unit, however, is composed of distal mudstone and siltstone rich

0 (from left to right) the total slt vf. f m c vc. the lower energy flow deposits. See table 1 and 2 for turbidite deposits. Finally, the Lower Castell Mudstone Bedset, this consists of cob peb

boul (ppm), K (%), U (ppm), Th mud gran facies scheme and facies associations respectively. A- background sedimentation, like the Caerhys Shale. However, the Th/K plot (Fig. 4) (ppm), the ratio of Th/K and authigenic uranium (UAut.). C locations are found in the sedimentological log within shows that these Castell mudstones may be more proximal in origin. Not all data has been presented here for the sake of figure 2. brevity. The sedimentary log displays calciclastic 3m debrite and hybrid co-genetic deposits the colours used for the log correspond with those within the Code Associations Facies Relationships Interpretation Discussion and Conclusion palaeoenvironmental interpretation in figure 5. The D.Mud. Very fine grainsize M.M., Low energy Pelagic Mud. sedimentary log is complemented with photographs of black mudstones, Pl.M. deposition. The geology of the Castell Limestone Formation consists of facies and various sedimentary features shown within low flow regime calciclastic debrite deposits and mudstones. The system was the outcrop. The sedimentological and spectral gamma structures ray data is correlated scales change where indicated. P.D.Slt. More proximal low M.Slt. Low energy More proximal than deposited as a succession of debris flows and co-genetic or hybrid 2m energy deposition deposition. D.Mud. but still distal of silts and low marine low energy flows. These deposits represent highstand shedding of a carbonate Unit Total[ppm] K[%] U[ppm] Th[ppm] Th/K U (Aut.) energy structures deposition. factory, this is shown in the decreasing trend in the Th/K scatter plat A 1216.11 3.04 3.49 14.39 4.73 C.Slt. Calcium carbonate M.Lst., Calcium Low energy marine B1 952.60 2.44 3.15 10.25 4.21 -0.26 precipitation C.Slt. Carbonate deposition above the data. This correlates to the global eustatic transgression and B2 1213.12 2.39 6.06 11.85 4.99 2.10 precipiatation C.C.D. subsequent highstand leading up to the massive regression during C 844.36 1.62 3.94 9.30 5.81 0.84 in low energy D 1025.31 2.21 4.22 11.41 5.25 0.42 deposition the early Sandbian. The palaeoceanic geochemical data from this M.F. High energy Bre Waining flow, Sub-aqueous mass time period suggests that this eustatic fluctuation in sea level may be 1m deposition, with decreasing flow deposits above Table 2 - The statistical mean for each facies belt and large clasts, rip-up energy up the C.C.D. as the correlated across the globe by using isotopic data, namely the each specific radioelemental emissions value. clasts and normal section facies contain 87Sr/88Sr and the 13C data. These isotopes produce anomalous trends Authigenic uranium does not have a mean value within grading. association. limestone clasts. in the data around the Darriwilian/Sandbian boundary, massive facies belt A as there is only one value that suggests 87 86 any is present. A – Caerhys Shale Formation, B1 – decrease within the Sr/ Sr data and the Middle Darriwilian Isotopic Lower Castell Mst. Bedset, B2 – Lower Castell Lst. Table 3 - Facies associations of the Castell Limestone Carbon Excursion (MDICE). 0m Bedset, C – Middle Castell, D – Upper Castell. Formation, facies codes correspond to those in table 1.

References Acknowledgements

Faculty of Natural Sciences