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Jl geol. Soc. Lond. Vol. 136, 1979, pp. 137-145, 1 fig. printed in Northern .

Ordovician and Silurian changes insea level

W. S. McKerrow

SUMMARY: Depth-relatedbrachiopod-dominated benthic communities can providewell- dated indications of change in relative sea level, especially if they OCCUT in tectonically stable regions. Eustatic changes in sea level occurred over short (less than 1-3 m.y.) intervals at the end of the Llandeilo, in the late Ashgill and at the base of the Upper Llandovery; they may be related to changes in the size of the icecap in Gondwanaland. In the Llandovery and in the late Silurian, there are periods of more prolonged eustatic changes in sea level. Other changes are be with shown to restricted to a sinaleI continent, and suggest vertical movements very little tilting.

Cycles of changingsea level with verydifferent migrating shorelines; these criteria have been used to periodicityhavebeen recognized during the some extent in the construction of Fig. 1. But the most :from the 100-200 m.y. cycles of Sloss useful data in thelate and Silurian are (1963, 1972) to the oscillations during the obtained from depth-related brachiopodcommunities; whichmay havebeen as short as 10,000 . The not only do they reflect relative changes in depth, but LowerPalaeozoic graptolite zones have anaverage most can be precisely dated. duration of from 3 m.y. (in the Ordovician) to 1 m.y. During the Ordovician, the gradually (in the Silurian), so that events of this duration (which replaced the as the dominant macrofauna in is about the same as the whole of the Pleistocene) are shelf environments(Cocks & McKerrow1978), and theshortest that can be correlated accurately. It is they continued to be prominent throughout the level- such 1-3 m.y. events,together with some more bottom shelf duringSilurian andDevonian times. prolonged movements, that are described here. Ziegler (1965) recognised five -dominated Previous assessments of Lower Palaeozoic changes communities in the early Silurian, each named after a in sealevel have normally been based on theareas typicalgenus: , Eocoelia, Pentamerus, Strick- covered by seaon palaeogeographic maps. This landia, and Clorinda. In the Welsh Borderland, con- methodhas three serious limitations: (1) the uncer- temporary communities map out in bands parallel to tainty of the positions of shorelines; (2) the difficulty in the shore in the E, and parallel to the margin of the dating littoral deposits; and (3) thefact that most maps in the W. The samecommunities occur in covering large areas do not show very fine time divi- the samesequence (from shallow to deep) in the sions. Provided that regions subject to repeated local AppalachianBasin (Ziegler & Boucot1970), New tectonic activity areavoided, it is preferableto de- Brunswick (McKerrow & Ziegler 1971), and in Iowa scribechanging environments in particularstratig- (Johnson 1977a). The width of shelf occupied by each raphicsequences. In this paper,many regions (like community varies from 5 km to over 100 km depend- eastern Australia) have been omitted because the data ing on the contemporary topography of the sea floor; availableshow no simultaneous changes in depth therefore their distributions are not directly related to from one sedimentary basin to another. In these reg- distance from shore. Nor are the communities directly ions, it is notpossible to distinguish the different related to sediment grain-size: apart from certain car- causes of depth changes frbm the available published bonates(where other communities are present), the accounts. brachiopodcommunities recognized by Zieglereach This paper is apreliminary attemptto look at occur in a variety of sediments. Those in shallow water world-wideOrdovician and Silurianchanges in sea can be found in conglomerates, sandstones and , level. It is hoped that it will encourage further work on whiletowards the deeper environments the coarser similar lines, especially in the regionsnot covered grain-sizes are progressivelyeliminated, so thatthe here. deep shelf Clorinda Community normally only occurs in fine sand and . Depth-related brachiopod In the late Silurian, the successors of the Llandovery communities communities have been recognised (Calef & Hancock 1974; Cocks & McKerrow 1978). These communities Changingfacies can give arough guide to changing containmany of the samegenera as the equivalent depths of sea,and so canstratigraphic breaks and communities in theearly Silurian, but some forms becomeextinct and others change in theirrelative 0016-7649/79/03004137$02.00 abundance through time. A succession of communities @ 1979 The Geological Society in thesame environments through time has been

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/2/137/4885914/gsjgs.136.2.0137.pdf by guest on 28 September 2021 138 W.S. McKerrow termed an ecogroup (Cocks & McKerrow 1978); the wanalandthroughout the Palaeozoic. These last two ecogroups are indicated on the columns of Fig. 1. columns were selected to show the stratigraphic posi- The correlation of thesebrachiopod communities tions of the tillites; all the other columns were chosen with depth of water is a theory not easily accepted by (a) because they were in relatively stable areas (where many ecologists who work on Mesozoic, Cenozoic or local movements did not obscure other changes moderncommunities. But it should be realized that in waterdepth), and (b) becausethere was reliable (a) it is onlylevel-bottom communities on clastic information on stratigraphic age and relative depth of substrates that are under consideration, and (b) that water.Western parts of NorthAmerica and all of during the late Ordovician and Silurian, brachiopods Australiawere omitted for one or both of these makeup the majority of thepreserved . (No reasons. doubtthere were manydeposit-feeding worms or Thedata from Poland (Column 1) are based on other animals which were related to the type of sedi- stratigraphic breaks and on influxes of coarser sedi- ment, but they are not preserved or are quite rare). ments recorded in the Ordovician and Lower Silurian Althoughbrachiopods are suspension-feeders, like on the eastern margin of the Russian Platform (Tomc- manymodern molluscs,they do nothave the same zyk 1971; Tomczyk & Tomczykowa 1976). The Upper complex filter-feeding mechanism, and it has recently Silurian and Lower data have been inter- beenshown (Steele-PetroviC 1975) thatthey have a preted from the faunal successions at Bostow (Tomc- much greatertolerance to mudthan suspension- zyk et al. 1977). The Ashgill and Llandovery fluctua- feedingbivalves. Many Silurian brachiopods had no tions are followed by Upper Silurian graptolitic shales, functional pedicles and restedon onevalve; those with which represent environments too deep to record any pediclesonly needed a shell fragment for pedicle small changes in sea level, but the incoming of some attachment in order toflourish on a muddy substrate. brachiopods in the Pridoli reflects a moderate shallow- The principal ecological factors controlling the dis- ing of the sea. tribution of Silurian brachiopods were (a) the greater Inthe Balticisland of (Column 2), the abundance of food (probably largely phytoplankton)in oldest beds exposed are of late Llandovery (C,) age the shallower environments (Hancock et al. 1974); (b) and contain a Clorinda Community, which represents greaterfluctuations in avariety of physicalfactors deep shelf environments. Earlier beds are graptolitic (sedimentsupply, temperature, storm effects) in the (Thorslund & Westergard 1938), suggesting still shallowerenvironments. The first factorwould have deeper conditions. The Wenlock and Ludlowcom- beenadvantageous to Lingula,Eocoelia, andother munities(Hurst 1975a) occur in formationswhich shallow water forms, while the second would provide appearto be diachronous, and there may besome morestable conditions for the deeper water genera lateral variation at any one time (hence the ellipses in (like Clorinda). Fig. 1, Column 2). There does, however, appear to be Upper Ordovician communities have beendescribed ageneral shallowing during the Wenlock(see also froma few areas in NorthAmerica (Bretsky 1970) Laufeld er al. 1975), followed by deeper conditions in and Britain (Hurst & Hewitt 1977), but much of the the basal . The youngest beds on Gotland show Ordovician data (Fig. 1) has been compiled from facies a return toshallow environments in mid-Ludlow time. changes (including some faunal changes) and sedimen- TheOrdovician in the Llandeilo-Llandoveryarea tary breaks. (Column 3) includesgraptolitic shales in theLower In the Silurian, brachiopod lineages can be used for Llanvirn, followed by shallow shelf sediments in the correlation (usually in terms of the standard Llandov- Upper Llanvirn and Lower Llandeilo (Williams 1953). ery formations: AI, A2, etc.), so that many brachiopod Progressively deeper environments are present during collectionscan be datedeven though they do not the later Llandeilo and basal Caradoc. TheLongvillian contain graptolites (Ziegler 1966; Cocks et al. 1971; andMarshbrookian stages are represented by mid- Ziegler et al. 1974). The stratigraphic basis for these shelf facies; these are succeeded by mudstones which correlations is indicated on the left of Fig. 1. are mostlyunfossiliferous, but which haveyielded some deep water brachiopod faunas of Ashgill age. A few miles to the N, towards Llandovery, the shallow Evidence for changes in sea water basal Silurian rests unconformably on the Upper level to the E of the Iapetus Ordovician, so there is evidence of uplift in the late OCean Ashgill (perhaps within the G. persculptus Zone). The Llandovery faunas of the type area show 4 episodes of In Fig. 1, the 13 columns represent sections from 3 deepening (from Pentarnerus Community to Stricklan- Lower Palaeozoic continents. Columns 1 to 7 lay to dia or Clorinda Communities) in AZ,B3, and basal Cl, the E of the (or Proto-Atlantic Ocean). CS, and C, (Cocks 1971). The Upper Silurian beds in Columns 8 to 11 were part of North America (to the the Sawdde Gorge (between Llandeilo and Llandov- W of theIapetus Ocean). North Africa and South ery) show progressive shallowing during the Wenlock America (Columns 12, 13) were both part of Gond- fromthe Dicoelosia Community (in the Clorinda

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Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/2/137/4885914/gsjgs.136.2.0137.pdf by guest on 28 September 2021 140 W. S. McKerrow Ecogroup)tothe Salopina Community (in the CambrianandOrdovician fauna1 relationships Eocoelia Ecogroup) followed by a sudden deepening (McKerrow & Ziegler 1971, 1972). At Arisaig (Col- (tothe Dicoelosia Community) in thebasal Ludlow umn 7), late Ordovician volcanics are followed uncon- (Calef & Hancock1974). The Ludlowbeds show a formably by acomplete succession from the Lower progressive shallowing upwards into the fluvial facies Llandovery to the Gedinne (Boucot et al. 1974). The of the . greaterpart of this sequencecontains brachiopods In the May Hill area (Column 4), Middle Llandov- (Watkins & Boucot1975), assigned to the Eocoelia ery beds rest on volcanics of the ? Hunt- Ecogroup, though deeper environments are indicated ley Quarry Beds. The sequence of Silurian brachiapod by some shales with sporadic graptolites in the Middle communities(Ziegler et al. 1968; Calef & Hancock Llandovery and the Lower Ludlow, andby the preval- 1974; Hurst 1975b, 1975c; Hurst et al. 1978) shows a ence of Isorthis macadamensis in the Lower Wenlock. remarkable parallelism tothe Llandovery-Sawdde Shallower environments occur at the end of the Lud- area except in the early Wenlock, where oscillations low (wherea bed with subaerialconcretions is are associated with the local development of sands in present), and in the Gedinne (where there is a transi- theWoolhope ; these are perhaps indica- tion through the Lingula Ecogroup up into beds with tions of local uplift along the Malvern Line (Hurst et freshwater fish). This sequence thus reflects the trans- al. 1978). gressionsseen in theareas so fardescribed which In (Column 5), the type Caradoc sequ- occur in the basal Llandovery, the basal Wenlock and ence (Dean 1953) shows synchronous depth changes the basal Ludlow, but the fluctuations seen elsewhere with the Caradocof the Llandeilo-Llandovery area; the in timeshave not been recorded. However, below faciesshow a progressive deepeningfrom the basal the first occurrence of Upper Llandovery fossils Costonian tothe LowerLongvillian, thena small (Eocoelia hemisphaerica of Cl-* age)there are over disconformity is followed by renewed deepening until 50 m of poorly exposed unfossiliferous shales (Boucot sedimentation ceases afterthe Onnian. There is no et al. 1974); a change of sea level during Cl time may Ashgill in Shropshire; and the Ordovician rocks were yet be recognized in Nova Scotia. folded and cleaved prior to the Llandovery transgres- sion. The Siluriancommunities S of theLongmynd Evidence for changes in sea level show a progressive deepening from B, to C, (Lingula to Stricklandia Communities);and a shallowing to to the W of the Iapetus Ocean Pentamerus Community depths in Cz is followedby deeper communitiesuntil thelate Wenlock regres- During the and most of the Ordovician, the sion(Ziegler et a!. 1968;Cocks & Rickards1969; Iapetus Ocean was wide enough to separate the ma- Calef & Hancock 1974; Hurst 1975b, 1975~;Hurst et jority of benthic marine animals (McKerrow & Cocks al. 1978).Like other areas to the E of theIapetus 1976; Ziegler et al. 1977). The distribution of these Ocean, the basal Ludlow of Shropshire shows a sud- fossils shows thatScotland and NW Ireland were den deepening;this is followed by a gradual regression attachedto North America on the W side of this upinto fluvial (Old Red Sandstone)facies in the ocean. F’ridoli (Downtonian). In the Girvan area (Column 8), atransgression in Inthe Lake District (Column 6), the Borrowdale the N. gracilis Zone is followed by deep water facies VolcanicGroup (of Llandeiloage) is followed by throughout most of the Caradoc, though there is some intermittent deposition during the Caradoc and Ash- northward migration of facies during this time (Wil- gill (Ingham & Wright1972). The bestevidence for liams 1962).The basalconglomerate of theupper lateOrdovician changes in sealevel in theN of WhitehouseFormation suggests local uplift in the England comes from the transgressive shoreline which basalAshgill, but deeperwater conditionsthen re- migrated southwards from theLongvillian to the Caut- turneduntil the , whenshallow water leyan (Ingham & Rickards 1974). In this area there is brachiopodsoccur in the HighMains Sandstone awidespread stratigraphic break just prior tothe (Whittington 1972, pp. 51-2). The Lower Llandovery Hirnantian (Ingham & Wright1972, fig. 8). The starts with shallow water facies resting disconformably youngestAshgill beds pass up conformably intothe on various Ashgill formations (Cocks& Toghill 1973). Basal Beds of the Silurian, which are and Thesubsequent Llandovery faunas indicate deep mudstones with a mixed shelly and graptolitic fauna, water environments, except for a slight shallowing in probably of G. persculptus Zone age (Ingham & Ric- B, and a more significant break in Cl-3. The youngest kards 1974, p. 36). The subsequent Silurian beds are beds in the Silurian of Girvanare early Wenlock all graptolitic, or turbiditeswith drifted shells, and shallow water sandstones. they thus do not reflect any sea level changes (hence In Galway and South Mayo (Column 9), graptolitic the dashed line in Column 6). Llanvirn shales are succeeded by shallow water sedi- NovaScotia is consideredto lie onthe eastern ments and then fluvial beds (Williams 1972; McKer- margin of the Iapetus Ocean,mainly on the basis of its row & Campbell 1960). Sedimentation did not resume

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/2/137/4885914/gsjgs.136.2.0137.pdf by guest on 28 September 2021 Ordovician and Silurian changes in sea level 141 in Galway until theUpper Llandovery,when the 1977a,19776). The late Wenlock and ?Ludlow fluvial Lough Mask Formation is followed by a sequ- GowerFormation of Iowaincludes pentamerid ence withprogressively deeperbrachiopod com- brachiopods (Berry & Boucot 1970), which may indi- munities. South of Killary Harbour, the Wenlock con- cate slightly deeper environments (Calef & Hancock sists of deep water turbidites followed by a regressive 1974), but some reef facies are also present. Thus in sequence(Laird & McKerrow1970). Thetectonic Iowa, we see a lowering of sea level in the late Ashgill instability of this region (like many others not men- and a short rise in Cl, both of which are recorded on tioned in this paper) thus precludes the recognition of the other side of the Iapetus Ocean, while the changes most of thesmaller changes in sea level. Thelate in C, and C, are only seen in North America. Llandoverytransgression is the onlywidespread change in sea level reflected in western Ireland. In the western part of New York State (Column lO), Gondwanaland tillites to the Wof the tectonically unstable Appalachian belt, there is an early Caradoc transgression over the Pre- In Morocco there was epeirogenic uplift at the end of cambrianrocks of theAdirondack Mountains the Llanvirn, at the base of the Upper Caradoc, and (Neuman 1976; Sweet & Bergstrom 1976, p. 133). In within the Upper Ashgill (Destombes 1976). The late the Ashgill,after thecompletion of theTaconic Ashgillregression can be correlated with extensive , there is a marked regression (Bretsky 1970) tillites in SouthernAlgeria (Bennacef et al. 1971). which has been linked with the contemporary North Glacial and glacially derived deposits have been traced African glaciations (Berry & Boucot 1973a; Sheehan across northern Africa, from Mauritania in the W, to 1973;Dennison 1976). West of theAdirondacks, a Ethiopia and Arabia in the E (Berry & Boucot 19736, basal Llandovery (A,) beach deposit rests disconform- p. 7).Glacially derived material has also been re- ably on Ordovician beds. This is followed by a Cryp- corded in Spain and Normandy. tothyrella Community (of the Eocoelia Ecogroup) of Allknown shallow shelf successions in North AZ age; and then (from late A, until basal B,) there Americaand northern Europe showa stratigraphic are nuculoid, Lingula and Arthrophycus Communities, break during the late Ashgill. The timing suggests that which are all assigned tothe Lingula Ecogroup this widespread break is related to a drop in sea level (Ziegler & Johnson1976; Johnson 19776). The re- associatedwith an icecap in northernAfrica. Pleis- mainingLlandovery communities are all assigned to tocene fluctuations in sea level have been estimated to the Lingula or Eocoelia Ecogroups, except for a Pen- have been as much as 200 m (Johnson 1977b, p. 113; tamerus Community in Cl and in C,, and a Clorinda Milliman & Emery 1968). Changes of this magnitude Community in c6. The basal Wenlock of western New wouldhave been sufficient to provide therecorded York State shows a marked shallowing, with a return breaks in shallow water Ashgill sequences. of an Eocoelia Community.Boucot (1975, p. 260) It is noticeable that the curves (Fig. 1) show an early recorded the Striispirifer Community from the Roches- Llandovery transgression following immediately after ter Shale, which suggests that the Srricklandia Em- the late Ashgill regression. This can be correlated with group was present in the early Wenlock. The succeed- theend of glacial conditions in southernAlgeria, ing Lockport Group (late Wenlock to Ludlow) was all wherenormal marine environments (with graptolitic deposited in shallow to mid-shelf environments (A. M. shales) had returned by the Middle Llandovery (Berry Ziegler 1978, pers. comrn.); the basal Ludlow deepen- & Boucot 19736). However, all the ice may not have ing seen on the eastern side of the Iapetus Ocean is melted.In South America, the Sacta Formation of absent in New York. The (of Pridoli southernBolivia and the Mecoyita Formation of age) contains a Lingula Community (Boucot 1975, p. northernArgentina contain tillites (Berry & Boucot 24). 1972); both these formations occur above fossiliferous The N. gracilis Zone is marked by awidespread late Ordovician beds, and the former lies conformably transgression in the central parts of the United States below fossiliferous Upper Llandovery beds. With the (Ross 1976), and this is reflected in Iowa (Column 11) movement of the polefrom northernAfrica, in the by the deposition of the St. Peter Sandstone (Sweet & late Ordovician, to South America in the early Devo- Bergstrom 1976,p. 133).In the basalAshgill, there was nian (Briden et al. 1974), the ice may have remained another transgression westwards from the Mississippi longer in South America than in northern Africa. The River,which reached its maximum in the Middle Cl changes in sea level consist of a short rise followed Ashgill(op. cit., p. 131). InIowa, the Ashgill Ma- by a drop. This canonly be explained by ice if the quoketa Shale was denuded (but not folded) prior to icecap first diminished and then increased. the basalLlandovery; Johnson (19776) recorded a The late Silurian sequences of both northern Africa relief of 45 m on the eroded topof this formation. The andSouth America (Columns 11 and12) show a subsequentLlandovery bedsinclude well- a general regression. This is marked by a retreat of the documented series of communities, which show a re- shoreline from southern Algeria, in the Middle Llan- markableparallelism with those in New York, with dovery, to northern Morocco, in the Gedinne (Berry peaks of increased depth in Cl, C,, and c6 (Johnson & Boucot 19736, fig. 1).

2

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/2/137/4885914/gsjgs.136.2.0137.pdf by guest on 28 September 2021 142 W.S. McKerrow Summary and conclusions spreading, or perhaps both. Several types of sea level changes can be recognizedin 3. Movements confined to a single continent: the Ordovician and Silurian: (Q) a drop in sea level E of the Iapetus Ocean in the Longvillian, followed by a rise in the late Caradoc. 1. Short pulses (of around 1 to 2 m.y.) occurring on (b) rises in sea level W of the Iapetus ocean in C, and all the continents studied: C,, followed by a drop in the early Wenlock; (Q) a rise in sea level in theLlandeilo and basal (c) a rise in sea level E of the Iapetus Ocean in the Caradoc; early Wenlock; (b) a drop in sea level in the late Ashgill, followed by (d) a drop in sea level E of the Iapetus Ocean in the a rise in the basal Llandovery; late Wenlock, followed by a rise at the base of the (c) a rise in sea level in the basal Upper Llandovery Ludlow. (Cl), followed by a drop in late C, or in C,. It is not at all clear how large continents could have These short pulses may be linked with changes in the moved vertically withapparently very little tilting; size of the ice cap in Gondwanaland, where tillites of however, this is what the evidence suggests. Sloss & Ashgill andprobably Llandovery ages are known. Speed (1974) invokedtransference of asthenosphere They are perhaps too rapid to be ascribed to changes melt from below continents (or below oceans) to exp- in oceanicspreading rates, though Hays & Pitman lain such events, but some of the movements (e.g. the (1973)demonstrated that rapid spreading at mid- basal Ludlow deepening in northern Europe) seem to oceanic ridges might cause changes in 1 or 2 my. (see have been too rapid for such an explanation. also Cooper (1976) for a good summary of the relev- literature). 4. Local changes in depth, which are probably due to local uplift or downwarping.These are generally 2. Prolonged eustatic changes in sea level: veryprevalent; the areas represented inFig. 1 have (Q) general a riseseain level through the been selected to avoid such local earth movements as Llandovery; far as possible. (6) a general drop in sea level during the Ludlow, and perhaps continuing through the Pridoli. ACKNOWLEDGEMENTS.I would like to thank Dr. L. R. M. It is notpossible atpresent to determine whether Cocks,Dr. J. M. Hurstand many Oxford colleagues for these were due to ice or to changes in rates of oceanic useful discussions in the preparation of this paper.

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Read 15 February 1978; received 15 March 1978; revised typescript received S May 1978. WILLIAMSTUART MCKERROW, Department of Geology & Mineralogy,Parks Road, Oxford OX1 3PR.

Discussion have omitted many areas from discussion in my paper DR M. G. BASSETTwrites: I am pleased thatDr for just this reason. Another criterion for selection has McKerrowhas drawn attention to the differences in been local consistency: if several different sections in scale at which ancient sea level changes can be recog- thesame regionshow changes in depthat different nized,both in a vertical and lateral distributional times, then local tectonic instability seems probable. sense. In particular he has pointed out in a number of For example, in Pembrokeshire, there are many differ- cases the important role that local tectonism can play ences between nearby Ordovician and Silnrian sequ- in causing changes, and that in some cases these may ences, and many of these appear to be due to local be imposedon or intercalatedwith eustatic andtor tectonic movements or to local volcanic activity. other regionalmovements. In recentyears ‘recogni- With regard to the supposed early Ludlow regress- tion’ of eustatic changes in various parts of the geolog- ion in Pembrokeshire, it would appear (Walrnsley & ical column has become quite fashionable, butin many Bassett 1976) that all thepost-Llandovery marine cases such ‘events’ are based on widely scattered evi- beds at Gateholm, Marloes and St. Ishmaels are likely dence withlittle accurate control on correlation be- to be of Wenlockage, andthus the transition from tween different areas. Such correlation is imperative if marineto non-marine (mainly fluvial) environments eustatic movements are to be invoked. occurred before the end of the Wenlock. This region I find that many of the Ordovician-Silurian sea level was probably sufficiently high above sea level for the changesdescribed by Dr McKerrowcould be inter- basal Ludlow deepening (seen elsewhere in Wales and preted in one or more ways, but in most cases it is a the Welsh Borderland) to go unrecorded in the local question of degree or scale. However, I would like to stratigraphic record. Similarly, the latest stratigraphic questionhis interpretation at the Wenlock-Ludlow work in southern Norwayshows thatthere is no boundary, where his diagram would suggest deepening definite evidence of marine Ludlow in this area. The overa fairly wide area. I haveelsewhere (Bassett regressive marine successions are probably all of Wen- 1976) drawn attention to the fact thatmany sequences lock age;there is no evidence of anybasal Ludlow in the Anglo-Baltic areasuggest widespread regression regression. and not transgression at this level(e.g. Pembrokeshire, In , the latest work on the Silurian by Kaljo ,Estonia), and in mostcases correlation is & Jurgenson (1977, p. 148) statesthat there are 3 considered to be accurate to at leastwithin a graptolite major regressive sedimentarycycles, which are interru- zone. In this respect I consider the deepening situation pted by relatively short transgressions. One of these in southern Wales and the Welsh Borderland to be an cycles endsat the Wenlock-Ludlowboundary. Al- effect of fairly localtectonic warping, whereas Dr though thereare no very great differencesbetween McKerrowpresumably sees it aspart of abroader their lithofacies maps for the lateWenlock and for the pattern. Would he care to comment, and also would Ludlow, it would appear fromtheir paper that the heoutline hiscriteria for correlation at this level? basalLudlow was thetime of one of theirshort With specific regardto his diagram,would he also trangressions.Their maps (unlike thedata in my comment on the Wenlock-Ludlow transition on Got- paper) do notshow the changing patternzone by land, which he includes in his deepening sequence; this zone; in their paper, data from several groupsof zones is another case in which I have previously suggested are lumped together. that shallowing took place at this level, reflecting local, In Gotland,a widerange (from deep to shallow differentialtectonism, and this interpretation is well water) of communities occur in the late Wenlock Halla supported within theBaltic Basin by thedetailed Limestone and Mulde Marl, with a predominance of evidencefrom Estonia where tectonic controls on shallowercommunities occurring towards the NE sedimentation have been well documented (e.g. Kaljo (Hurst 1975~).The succeeding Klinteberg Limestone, 1970). If Dr McKerrow maintains his recognition of a which contains local shallowwater communities, is deepening Wenlock-Ludlowsequence onGotland, diachronous; it is followed by the Hemse Beds which would he again care to spell out the criteria, including alsohave adiachronous base. However, thedeep his basis for correlation at this level. water communities in the Hemse Beds all appear to be of Ludlowage, and they underlie younger shallow In reply, DR MCKERROWwrites as follows: Dr. Bas- water beds also of Ludlow age (Hurst 1975~).While I sett emphasizes the importance of using precisely cor- stand by thestatements on Gotland in my paper,I relateddata for recognisingchanges in sea level. I would agreethat more workneeds to bedone in

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/136/2/137/4885914/gsjgs.136.2.0137.pdf by guest on 28 September 2021 Ordovician and Silurianchanges in sea level 145 Gotlandto establish the preciseages of the very References variable and diachronous formations. The evidencefor synchronous changes in depth BASSETT,M. G. 1976. A critique of diachronism, community close to the base of the Ludlow in Wales and England distribution and correlation atthe Wenlock-Ludlow hasbeen well summarized by Hurst (1975b), and boundary. Lerhaia 9,207-18. much of thedetailed evidence is summarized HURST,J. M. 1975a. Some observations on brachiopods and elsewhere(Hurst et al. 1978). Severalsections in the level-bottom community ecology of Gotland. Geol. Wales and the Welsh Borderland show a change from Foren. Srockh. Fork 97, 25M4. the veryshallow Salopina Community to thedeep - 1975b. The diachronism of the WenlockLimestone. Lethaia 8, 301-14. shelf Dicoelosia Community in the basalLudlow. -, HANCOCK,N. J. & MCKERROW,W. S. 1978. Wenlock Many of these transitions occur in sections with grap- stratigraphy and palaeogeography of Wales andthe tolites, indicating thatthey are close in timeto the Welsh borderland. ploc. geol. Ass., 89, 197-226. boundarybetween the P. ludensis and P. nilssoni WO,D. L. (ed.). 1970. The Silurian of Estonia. 343 pp., Zones. I consider that these fauna1 changes represent 16 pls. Tallinn. [In Russian with extensive English sum- such a large change in depth of water that they are mary.] more likely to be synchronous than diachronous, espe- -& J~RGENSON,E. 1977. Sedimentary facies of the East ciaily over the short distances involved in the Welsh Baltic Silurian. In KAUO, D. (ed.) Facies and fauna of Borderland. Admittedly, the change in faunas is sud- the Baltic Silurian. Academy of Sciences of the Estonian den in some sections and gradational (through -a few S.S.R., Tallinn, 122-48. WALMSLEY,V. G. & BASSETT,M. G. 1976. Biostratigraphy metres) in others (seeBassett 1976), but this can and correlation of theCoralliferous Groupand Gray surely be explained by slightly variable sedimentation Sandstone Group (Silurian) of Pembrokeshire, Wales. rates. Proc. Geol. Ass. 87, 191-220.

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