Eustatic sea-level patterns from the Lower Silurian

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Geological Society of America Bulletin Eustatic sea-level patterns from the Lower Silurian (Llandovery Series) of southern Norway and Estonia M. E. JOHNSON, B. G. BAARLI, H. NESTOR, M. RUBEL and D. WORSLEY Geological Society of America Bulletin 1991;103, no. 3;315-335 doi: 10.1130/0016-7606(1991)1032.3.CO;2

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Notes

Geological Society of America


Eustatic sea-level patterns from the Lower Silurian (Llandovery Series) of southern Norway and Estonia ß 1 G BAARLI° N 1

De artment

P

of Geology, Williams College, Williamstown, Massachusetts 01267

H NESTOR 1 »I t f Institute of Geology, Estonian Academy of Sciences, Tallinn, Estonia, U.S.S.R. M. RUBEL J D. WORSLEY Saga Petroleum a.s„ P.O. Box 490, 1301 Sandvika, Norway

ABSTRACT America. He followed workers such as Berry and Boucot (1973) in pointing out the global transgression recorded at the base of the Silurian, but also recognized a highstand in sea level recorded near the base of the Monograptus sedgwickii graptolite zone in the Aeronian Stage (Llandovery Series). Four sea-level cycles are widely recorded in the Llandovery Series of North America (Johnson, 1987). They include a highstand peaking near the Rhuddanian-Aeronian transition, the middle Aeronian highstand recognized by McKerrow (1979), an early Telychian highstand, and a late Telychian highstand peaking near the close of Llandovery time. The first, third, and fourth of these are biostratigraphically correlated with sea-level patterns preserved across the Yangtze platform of South China (Johnson and others, 1985). McKerrow (1979) included some data from Poland, but most of his European data was derived from the British Isles. Thick sequences of Lower Silurian strata bearing the same shelly faunas are well preserved in the Oslo region of southern Norway and the East Baltic region of Estonia (Fig. 1). These areas were part of the paleocontinent Baltica, and their surviving strata provide a suitable opportunity to further test the concept of Early Silurian eustasy.

This study demonstrates the utility of checking for eustatic sealevel events correlated between passive epicontinental seas and those on the margin of orogenic activity. Bathymetric profiles interpreted on the basis of stratigraphic data from the Lower Silurian (Llandovery Series) in five districts of the Oslo region of southern Norway are compared with similar data derived from five bore-hole sites in the Baltic region of Estonia. These regions preserve extensive marine strata deposited on the Silurian continent Baltica. The data are tested for intra-regional and inter-regional conformity. Despite the fact that the Oslo region was affected by the Caledonide Orogeny, the same four highstands in sea level were recorded there as on Estonia's comparatively stable platform. Expressed in terms of the established geochronologic standard and its faunal zones, coeval highstands in sea level occurred across Baltica during latest Rhuddanian time, in mid-Aeronian time (as marked by the basal Monograptus sedgwickii or Stricklandia lens progressa zones), in early Telychian time ( M turriculatus or S. laevis zones), and in late Telychian time near the Llandovery-Wenlock boundary. This timing corresponds to four highstands in sea level previously documented in North America and to at least three highstands found in South China. Claims for Early Silurian eustasy now rest on data collected from three independent paleocontinents. If the Llandovery Epoch lasted 10 m.y., then sea-level cyclicity was on the order of 2.5 m.y. A combination of tectono-eustastic and glacioeustatic causes probably was responsible for these cycles. INTRODUCTION Eustasy is a difficult concept to prove in practice, because the pattern of sea-level changes recorded on independent and globally dispersed continents must be demonstrably coeval. Pioneering work on Paleozoic eustasy was undertaken by A. W. Grabau (1936), who brought to the task an extensive knowledge of European, North American, and Asian stratigraphy. Some of the regional Silurian events he cited remain valid as eustatic markers (Rong and others, 1984), but not as he correlated them globally. The search for intercontinental Silurian patterns was adopted next by McKerrow (1979), who focused on Europe and parts of North

H. Nestor (1972) commented on the late Llandovery transgression in Estonia and its correlation in northern Europe. A preliminary report on Llandovery sea-level patterns in the Oslo region was made by Johnson and Worsley (1982), enlarging on a previous attempt by Seilacher and Meischner (1964). A large body of paleontological and stratigraphical literature concerning these regions has since accumulated. The object of this article is to compare the stratigraphic patterns of sea-level cycles recorded during Early Silurian (Llandovery) time in southern Norway and Estonia. Although faunas and some lithological facies are very similar, the depositional settings of the two regions contrast sharply. The Baltic platform was tectonically passive during the Silurian in comparison to Scandinavia, which was strongly affected by the Caledonide Orogeny. The following questions are addressed by this study. What is the nature and timing of sea-level cycles from the Llandovery Series of Estonia and Norway? How do these cycles correlate between the tectonically more passive and more active parts of the paleocontinent? Is it possible to corroborate the concept of Early Silurian eustasy by matching sea-level patterns from Baltica with those now understood from North America and China?

Additional material (one table) for this article may be secured free of charge by requesting Supplementary Data 9103 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 103, p. 315-335, 17 figs., March 1991. 315

316

Downloaded from gsabulletin.gsapubs.org on December 24, 2012 JOHNSON A N D OTHERS

TIME SCALE A unified time scale is necessary to distinguish sea-level events of local, regional, or intercontinental magnitude from one another. The many Llandovery brachiopods shared in common by the Baltic and Oslo regions

provide the basis for a compatible Estonian-Norwegian time scale. These include species or subspecies belonging to the genera Stricklandia, Costistricklandia, Borealis, Pentamerus, and Pentameroides. Chronologic subspecies of Stricklandia lens (including the form S. ultima now designated S. laevis) were defined originally by Williams

Figure 1. Scandinavia and part of eastern Europe (A), with more detailed maps of the Lower Silurian (Llandovery) in the Oslo region of southern Norway (B) and Estonia in the Baltic region (C).


317

EUSTATIC SEA-LEVEL PATTERNS, SOUTHERN NORWAY A N D ESTONIA

TAXA

MUTUAL RELATIONSHIPS

Pentamerus oblongus Stricklandia laevis Stricklundia lens progressa Borealis borealis osloensis Stricklandia lens intermedia Borealis borealis Stricklandia lens lens Stricklandia lens prima

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c) Joint Estonian - N o r w e g i a n d a t a Figure 2. Procedures followed in erecting a compatible time scale for the Baltic and Oslo regions: summary of Estonian faunal data and range chart (a); summary of Norwegian faunal data and range chart (b); summary of joint Estonian-Norwegian faunal data and common range chart.

(1951) from collections made in the type Llandovery district of Wales. Subsequently, Rubel (1977) demonstrated the occurrence of the same succession in Estonia, and Baarli (1986) did the same in southern Norway. The descendent genus, Costistricklandia, also occurs in Estonia (V. Nestor and others, 1978) and Norway (Baarli and Johnson, 1988). Another important evolving sequence concerns the BorealisPentamerus-Pentameroides lineage. Mark (1981) defined the subspecies B.

borealis osloensis as a transitional form between B. borealis and Pentamerus oblongus. The subspecies is not separated in our Norwegian data set, however, because Mark (1981) was unable to show the stratigraphic interval where the transition from B. borealis occurs. Passing through Pentamerus, the lineage's same end genus, Pentameroides, also occurs in Estonia (noted by Rubel, 1970a, p. 21; illustrated by Ivanowskii and Kulkov, 1974) and Norway (Baarli and Johnson, 1988). Although long

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JOHNSON A N D OTHERS

(1984). Adopted here in simplified form, their approach accepts biostratigraphic data only from objective sequences. Estonian data on Costistricklandia and Pentameroides thus must be excluded because subjective factors are involved in correlating small isolated outcrops to the nearest thick core sequence. Figure 2 shows three stages in the construction of an inter-regional time scale. Beginning with the Baltic region, the first question to be ap-

WENLOCK

outcrop sections embrace all of the lineage members in Norway (Baarli and Johnson, 1988), Pentameroides is known only from short, isolated outcrops in Estonia. This is also true for the Estonian Costistricklandia All of the other pentamerid and stricklandiid lineage members, however, are well demonstrated in Estonian sections represented by continuous cores. Theoretical principles leading to the construction of fossil-based time scales were discussed by Rubel (1978) and elaborated by Rubel and Pak

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1983, p. 25). Finally, the highstand which characterizes the upper part of the Vik Formation and the lower part of the Bruflat Formation occurred entirely within the range of Costistricklandia. The unconformity between the Bruflat and Brakseya Formations is shown near the LlandoveryWenlock boundary (Fig. 8), but its precise age is uncertain. The Skien District Cliffs and road cuts in the Lower Silurian are well developed in and around the city of Skien in the southern district of the Oslo region (Fig. IB). A partial stratigraphic log and bathymetric profile for this district (Fig. 9) reflect data collected on and above Blabaerstien (a city street). Our log begins at the base of the Rytteraker Formation; no recent compilation of data is available for the underlying Saelabonn Formation. Work on the Vik and overlying formations is in progress (Cocks and Worsley); our log ends within the lower part of the Vik Formation. The lower 25 m of the Rytteraker Formation are composed of thin interbedded green shales and limestone tempestites rich in crushed Pentamerus debris (Johnson, 1989). These layers represent a marginal environment for Pentamerus (transitional between BA3 and BA2). A shallow-water calcareous sand bar is found at the 30-m level (BA2). At the 35-m level, a persistent shale layer bears Clorinda, Stricklandia, Pentamerus, and Eocoelia. If the depositional environment is BA5, then all except Clorinda represent elements transported down slope. A shallowing trend is suggested by the occurrence of Pentamerus at the 45-m level, followed by a coral-stromatoporoid biostrome at the 60-m level. The basal meters of the Vik Formation record a deepening trend, with Pentameroides-rich layers (BA3) succeeded by Costistricklandia (BA4). Biostratigraphy (Fig. 9; Baarli and Johnson, 1988) suggests midAeronian and mid-Telychian highstands in sea level.

show comparative trends in bathymetric profiles without redrawing the sections against the framework of a common time scale (Fig. 10). According to the lateral relationship of benthic zones, the Oslo District sits at one end of a continuum in the deepest, most land-distal environment; the Skien District sits at the other in the shallowest, most land-proximal environment. This treatment shows a high intra-regional correlation among sealevel events. The first highstand in sea level was recorded just prior to the Rhuddanian-Aeronian transition in the Asker, Modum, and Ringerike districts. Only a hint of this event may be found in the Oslo District; it remains uninvestigated in the Skien District. A second highstand occurred in all districts except for the Oslo District. Its correlation is consistent with a position low in the Monograptus sedgwickii zone (which accounts for more time than any of the other graptolite zones in the Aeronian). Thus, the second highstand in sea level may be referred to mid-Aeronian time. A third highstand is well coordinated in the Oslo, Asker, and Modum districts and probably in the Ringerike District (where comparable biostratigraphic control is lacking). The timing of this event is early Telychian, correlative to the Monogratus turriculatus zone. Coeval data from the Skien District are inconclusive. Two possible events of coeval deepening may be referred to the upper Llandovery. One deepening is marked by the M. griestoniensis zone in the Ringerike, Oslo, and Skien districts, but data from this interval are missing in the Asker District. A subsequent highstand in sea level occurred in the Oslo and Asker districts during latest Telychian time. Biostratigraphic control from the Ringerike and Skien districts does not resolve whether this change was near the Llandovery-Wenlock boundary or somewhat later in Wenlock time; correlative data are unavailable in the Modum District. The two upper Telychian deepenings were regarded as one continuous eustatic event by Baarli (1990a), interrupted by progradation of a delta or shore from the present north or northwest.

Regional Summary for Southern Norway THE LLANDOVERY SERIES OF ESTONIA Stratigraphic columns for key sections in the Eve districts described above (Figs. 5-9) are drawn to scale. As some districts record relatively rapid deposition and others more normal deposition, it is impossible to

Lower Silurian (Llandovery) strata are poorly exposed in central Estonia, but between 100 m and 250 m of core have been recovered from

EUSTATIC SEA-LEVEL PATTERNS,24, SOUTHERN Downloaded from gsabulletin.gsapubs.org on December 2012

drilling sites across southern Estonia (Fig. 1C). The Estonian Silurian System has been studied for well over a century, but important strides have resulted from a program of subsurface stratigraphy pursued over the past 35 years (Aaloe and others, 1976; Kaljo, 1982). Much of the Lower Silurian sequence is composed of the marly Velise, more limestone-rich Rumba, mixed marly and calcareous Saarde, and shaley Ohne Formations, in descending order. The Ohne and Saarde Formations in southern Estonia are more central to the depositional basin. The Ohne Formation is replaced by the Varbola and Tamsalu Formations on the basin margin in

NORWAY A N D ESTONIA

327

central Estonia. The upper part of the Saarde Formation also has the same facies relationship with the Raikkula Formation on the basin margin. Nestor and Einasto (1977, 1982) demonstrated the physical bathymetry and related lithostratigraphy of the Estonian Silurian, based on the model of a land-hinged shelf associated with a slope and basinal depression. Fine-scale cyclic changes are detectable following the stratigraphic arrangement of five key lithofacies. The related bathymetries of brachiopods (Rubel, 1970b) and graptolites (Kaljo, 1978) provide important auxiliary guides to the reconstruction of sea-level events on a coarser

Figure 10. Comparison of sea-level curves from five districts in the Oslo region. The Skien District (right) is considered the most proximal to land; the Oslo District (left) is considered the most distal to land. The numbers below each profile represent benthic assemblage zones (after Boucot, 1975).

328


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