LOWER LIAS OF ROBIN HOOD'S BAY 111 



the very conspicuous scar Low Balk 0.10 



Euagassiceras resupinatum (Simpson) (21; CA 2942-62; PI. 1, fig. 5). 



422.1 Hard grey micaceous calcified shale, weathering brown 0.30 



Euagassiceras resupinatum (Simpson) ( 1; CA 2941 ). 



421.4 Hard partially calcified shale, micaceous and with strings of pyrites in places; a few harder calcified lenses near the top .. 0.91 



Arnioceras semicostatum (Young & Bird) (31; CA 2806-28, CA 2974-81), Euagassiceras resupinatum (Simpson) 

 (2; CA 2939-40), Euagassiceras sp. indet. (5; CA 2963-67). 



421.3 Shale; forms a terrace on the scar due to a harder band at the top 0.46 



421.2 Shale; forms a terrace due to a harder band at the top 0.69 



Euagassiceras sp. indet. ( 1 ; lost). 



421.1 Shale; forms a terrace due to a harder band at the top 0.91 



Euagassiceras sp. indet. (2; lost), Arnioceras sp. indet. (1; lost). 

 420 Hard grey flaggy limestone passing down into grey shale; slightly micaceous and pyritous; full of bivalves in places; 



forms a dip slope 25 m wide and a scarp face 0. 13 m high 1 .75 



419 Hard grey calcified shale, slightly micaceous; dip slope 1 m wide, and scarp face 0.13 m high 0.13 



418 Hard grey partially calcified shale; slightly micaceous; forms a terrace 4 m wide, ending seaward in a scaip face 0.46 m 



high; the lowest bed exposed at low water of the lowest spring tides 0.60 



LITHOSTRATIGRAPHY 



The Lower Lias of Robin Hood's Bay is 1 63.74 m thick and belongs 

 to the Redcar Mudstone and the lower part of the Staithes Sandstone 

 Formations. These names were introduced as formations by Powell 

 (1984: 53) and Howard (1985: 262), and details of their definitions 

 were given by Cox et al. (1998: 35,39). Fig. 18 is a complete vertical 

 section of the succession of the Lower Lias in Robin Hood's Bay, 

 showing the changes in lithology, the lithological divisions and the 

 main features of significance formed by the harder beds, some of 

 which have received formal names; bed thicknesses are drawn to 

 scale on this figure, giving a visual indication of the relative thick- 

 nesses of the subzones and lithostratigraphical divisions. 



Staithes Sandstone Formation (beds 591-601 

 higher) 



and 



Consists of sandstones, that are mid to pale grey, fine to medium- 

 grained, micaceous, with grey siltstone bands and some beds of silty 

 shales; nodules of argillaceous limestone occur in the shales and are 

 occasionally sideritic; there is much bioturbation, cross-bedding and 

 ripple marked bedding, especially in the sandstones. 



The type area is in Robin Hood's Bay. The formation is somewhat 

 transitional from the underlying Redcar Mudstone Formation, but a 

 convenient marker bed that defines the lower boundary just south of 

 Castle Chamber is bed 591, the Oyster Bed, which is a hard, 

 calcified, silty shale or argillaceous sandstone, containing many 

 oysters and other fossils. Above this level the amounts of silt and 

 sand are higher than lower in the sequence and hard and soft 

 sandstones are frequent. The formation has no subdivisions and 

 extends up into the lower half of the Upper Pliensbachian. The 

 Lower Pliensbachian (ie. Capricornus and Figulinum Subzones) part 

 of the formation is 12.74 m thick. 



Redcar Mudstone Formation (beds 418-590; 151 

 thick) 



m 



Consists of mudstones and shales, grey, soft and well-bedded, but 

 some beds are indurated due to calcification, and there are some 

 harder siltstones; thin beds of shelly limestone occur in the lower 

 part, and nodules of calcareous or sideritic mudstone occur, especially 



in the upper part; pyritized nodules or irregular aggregations of iron 

 pyrites occur at some horizons. 



Although the name is derived from the Lower Lias exposures at 

 Redcar to the north-west, and the lower boundary is defined in the 

 BGS Felixkirk Borehole (Cox et al. 1998: 35), the type section 

 consists of the whole sequence exposed in Robin Hood's Bay below 

 the base of the Staithes Sandstone Formation. Here the lithology is 

 more argillaceous than in the Staithes Sandstone Formation, though 

 different parts are variously more calcareous, siliceous, pyritous or 

 ferruginous, and they form the basis of the following four members: 



4. Ironstone Shale Member. 

 3. Pyritous Shale Member. 

 2. Siliceous Shale Member. 

 1 . Calcareous Shale Member. 



These members were introduced as 'Shales' by Buckman (1915: 

 61 ) for lithological divisions in Robin Hood's Bay below the 'Sandy 

 Series' (= Staithes Sandstone Formation), and he based them on 

 groups of the ammonite zones that he applied to the succession. His 

 zones can be linked to beds in his detailed sections, but they are not 

 the same as the ammonite zones recognized now, and in any case it 

 is not satisfactory to base lithological divisions on the ranges of 

 palaeontological zones. No formal definitions have been given to 

 these divisions, though Hesselbo & Jenkyns ( 1 995 : 114-135) applied 

 them informally to the succession, and placed boundaries between 

 them at appropriate levels in the succession, all of which are followed 

 here. The four members are defined formally here, and their type 

 sections are in Robin Hood's Bay. 



Ironstone Shale Member (beds 527-590; 62.73 m thick). Mud- 

 stones and shales, grey, soft, with some beds of micaceous silty shales, 

 many grey sideritic mudstone nodules, weathering red on the outside, 

 and a few calcareous mudstone nodules. The nodules are scattered 

 sparsely through the shale, but are more often developed at single 

 horizons, either scattered or as near-continuous beds. The distinctive 

 beds of red- weathering nodules have frequently been referred to as 

 'ironstones', but they are better described as sideritic mudstones. 

 The base is defined at the bottom of bed 527 in Robin Hood's Bay, 

 which is a prominent continuous bed of sideritic mudstone weather- 

 ing red-brown. Although sideritic mudstone nodules occur at several 



