Glo bal earlie r Ordavieian transgressions and regressions and their biological implications By RICHARD A. FORTEY Global marine transgressions and regressions serve to define the original Series into which the earlier part of the Ordavieian System was divided. The biological effects of these eyeles are variously, but simultaneausly expressed on what were independent continental blocks at the time. The faunal changes which occur at Series boundaries are as much a product of environmental shift as of evolution­ ary novelty. Scarcity of recoverable deep water facies from tectonic eauses and partly from lack of searching) during regressive phases has meant that "ancest­ ral" faunas have been generally overlooked, but they can be found in the cor­ rect sites in areas peripheral to former continents. Thus some of the "Llanvirn faunas " (transgressive) of Ordavieian Gondwanaland can be identified in Arenig off-shelf occurrences in peripheral sites, and the "Middle Ordovician" North American fauna has a progenitar in what is believed were earlier rocks in Spits­ bergen. Conversely, times of regression exposed offshore islands, and many (but not all) of these island faunas earrespond with regressive intervals on the plat­ form. Faunal interchange in relatively uniform deep water faunas may have pro­ ceeded in advance of major changes in provinciality, which are manifest when these faunas move shelfwards during transgression. These ideas are discussed in relation to trilobite and graptolite biofacies during the Tremadoc to Llandeilo. The eustatic changes could have been eaused by fluctuations of a Gondwanan Iee Sheet. R. A. Fortey, Department of Palaeontology, British Museum (Na tural His tory}, Cromwell Road, London SW 7 5BD, England. In recent years a number of authors have drawn various scales; those which are addressed here attention to the broad geological effects of are believed to have been major eustatic events transgressions and regressions during the Ordo­ independent of local tectonic circumstances. vician (Vail et al. 1977; Leggett 1978; Leggett Especially in mobile sites at active continental et al. 1981). Major sea level changes of this margins during the Ordovician , there may be type have been implicated in faunal changes camplex regional transgressions or local uncon­ which occurred during the Ordavieian (Shaw formities which may seem locally more signifi­ & Fortey 1977; Jaanusson & Bergström 1980; cant than the more general eustatic events, and Ludvigsen 1982). In this paper I attempt a brief which may operate to enhance or oppase such overview of the biological effects of transgres­ events. Criteria which may be used to distin­ sions and regressions during the earHer part of guish eustatic events of more than local signif­ the Ordovician . In a review of this length it is icance are judged to be particularly : not possible to give detailed documentation of all the assertions; I have concentrated on a l . That simultaneous regressions or trans­ number of critical instances which may be used gressions occur on what were separate con­ in support of the general picture. tinental blocks (i.e. belonging to separate lithospheric p lates), thus minimising region­ al influence. Recognition of global as opposed 2. That the events will be regionally correlat­ to local events able in a consistent way. A transgressive Regressive-transgressive events can operate on event, for example , may be expected to in- In Bruton, D. L (ed.), 1984. Aspeels of the Ordavieian System. 37-50. Palae­ ontological Contributions from the University of Oslo. No. 295, Universitets­ 37 forlaget. troduce sediments onto eratonic areas for the first time since the last transgresssion, and this should be directlyco rrelatable with events such as facies changes in peripheral ZONE Of CC!t'o!Tif'«JQUS ZONE OF ALTERI'\IATING UTHO - ZONE OF PlATFORM SEDIMENTATION AND 610fACIES NON - SEOUENCES eratonic sites where sedimentation is other­ wise continuous. (Fig. 1.) 3. Only truly off-shelf sites, which are gene­ rally developed in graptolite facies in the Or­ l dovician, are relatively immune from the ef­ fe cts of transgressive-regressive cycles. These . are to be taken as the standard when asses­ sing what mightbe "missing" in platform re­ ill!!!!�,��j�� gions. ·•. ·.· Both correlation between open-ocean facies andshelf faunas, and between widely separated Fig. l - Thesimp/est type of fa cies distribution pat· terns connected with transgressive and regressive eratonic areas - especially if they we re at diffe­ cycles. Platform sandstones to righ t grading rent palaeolatitudes - pose particular strati­ through Iimestones to deeper water shales. During graphic problems, a product of the more-or-less regressive phase unconfo rmities should coincide with exterior migration of limestone fa cies. patchy distribution of the biostratigraphic in­ dices. It is important to avoid circular argu­ ments (for example, making an a priori assump­ tion that two approximately coeval regressive sparse island faunas "r deeper water faunas plankton impinge on shelf on·shelf endemism high A. is land faunas ex t end plankton offshore on ly increose B . Fig. 2 - Cartoon representing so me of the effects prediered during times of transgression (A ) and regression (B) on opposing confinental blocks. 38 conlinental edge s were active margins in many place s. ASHGILL A. Transgression CARADOC During a relatively transgressive phase the fol­ lowing biological effects are predicted: (F i g. 2) LLANDEILO llANVIRN l. Flooding of eratonic areas will produce widespread shallow epieric seas. Spatial he­ 1974) ARENIG terogeneity (Eldredge and the "spe­ cies area" effect (Ludvigsen 1982) will play a part in inducing high speciatian rates in epicontinental areas. eratonic areas separa­ TREMADOC ted - especially by latitude - from their neighbours will generate endemic taxa. 2. As transgression proceeds, and especially in more exterior sites, previously extra-era­ Fig. 3 Times ofproposed maximum regression (ar­ tonic biofacies will be brought on to the rowed)- with hypothetical sea-leve/ curve to righ t. shelf. Such changes will be more or less dia­ chronous according to the rapidity of trans­ eyeles are exaetly contemporary just because gression (below). In some cases this may en­ they are regressive) by treating fauna! evidence tail displacement shelf-wards of faunas usual­ on its own merits, and separately. The earre­ ly living below the thermoeline, and may lations used here are , naturally, my own , and it produce extinctions of shelf forms. would be surprising if there were not areas of 3. Because deep water faunas are more inde­ disagreement with other specialists. I have eho­ pendent of continental boundaries, times of sen to treat the Tremadoc to Llanvirn in detail transgression may appear as times of provin­ because it is best known to me ; there have been cial breakdown. numerous recent studies on the regression asso­ 4. In tropical areas mound faunas will migrate ciated with the late Ordavieian gl aciation (Shee­ on-shelf. han 1973, 1975; Brenehley & Cocks, 1982) and 5. Island faunas will be generally rare r as off­ fu rther comment here would be nugatory . shore 'highs' are immersed. This will apply only in the broadest sense , because active Biologieal effeets of vulcanity will overtake transgressive events in some circumstances. transgressive-regressive eyeles As a basis for conside ring the effects of trans­ B. Regression gressive-regressive eyeles on marine organisms, assumptions are made concerning the distribu­ During a relatively regressive phase the follow­ tion of biofacies in a profile running from the ing biological effects are propose d: interior of crawnic areas to the open ocean (Fig. 2). This is developed from the trilobite­ l. Inte rior eratonic sites will either have strati­ hased biofacies/community-type analyses of graphic gaps , or, seawards, super- or infra­ Fortey (1975, 1980), Ludvigsen (1975) and tidal deposits poor in fo ssils. These might Cocks & Fortey ( 1982). Inner shelf-to-sl ope be dolomites in the tropical regions or Gres transects are interrupted in tropical latitudes by Arrnoncain facies (Dean 1976) at high lati­ a earbonate mound facies which does not have tudes. a counte rpart at higher latitudes. Volcanic is­ 2. On the eratonic interior, across such a re­ lands are introduced on the assumption that gressive phase , faunas will appear to dunge 39 with a taxonornie jump, which will not be so Times of regression an d transgression apparent in off-shelf faunas. 3. Retreat of biofacies seawards will mean that Using the criteria for recognition of worldwide the "ancestors" of the faunas which are transgressive-regressive events Iisted above, the found on-shelf during transgressive phases following times are suggested for the climaxes must be sought in peripheral sites. Because of eyeles (Fig . 3). such sites are narrow , and often involved in subsequent tectonism, or over-ridden by Basal Tremadoc nappes, they will be uncommon. 4. Conversely, regressive phases will tend to This is a time of regression ; its world-wide increase the incidence of island faunas (or character was suggested by Miller (1978), and faunas fringing microcontinents) by a greater reiterated on different evidence by Leggett et extension of surmunding productive shelves, al. (1981). In this paper, evidence of its effects and by bringing formerly submerged volca­ can be deduced from what happens around se­ nie islands to shallow sub-littoral depths . veral independent contioental areas at the time. These faunas will thus tend to belong in the eratonic "gaps"; exceptions are note d above . (a) On the Gondwanan margin of lapetus, in the 5. Mound faunas in the tropics will be near the Anglo-Welsh area , which embraces a eraton­ edge of shelf areas . Regressive phases will to margin profile , sedimentation across the coincide with times when allochthonous de­ Cambro-Ordovician boundary is complete bris-slides from such sites were at a maxi­ only in the peripheral area in North Wales re­ mum, as in the Cow Head Group, western cently documented by Rushton (1982).