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Ordovician and Silurian Changes in Sea Level 141 in Galway Until Theupper Llandovery,When the 1977A,19776)

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Ordovician and Silurian Changes in Sea Level 141 in Galway Until Theupper Llandovery,When the 1977A,19776) Jl geol. Soc. Lond. Vol. 136, 1979, pp. 137-145, 1 fig. printed in Northern Ireland. 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 Phanerozoic: from the 100-200 m.y. cycles of Sloss useful data in thelate Ordovician and Silurian are (1963, 1972) to the oscillations during the Pleistocene obtained from depth-related brachiopodcommunities; whichmay have been as short as 10,000 years. 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 brachiopods gradually (in the Silurian), so that events of this duration (which replaced the trilobites 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 brachiopod-dominated Previous assessments of Lower Palaeozoic changes communities in the early Silurian, each named after a in sealevel have normally been based on the areas typicalgenus: Lingula, 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 Welsh Basin 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 shales, 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 shale. 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 wanaland throughout 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 earth movements did not obscure other changes modern communities. 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 fossils. (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 Devonian 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 Gotland (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 shallower environments. The first factor would 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 Ludlow. 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
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