Ordovician carbonate mud mounds of the Baltoscandian Basin in time and space – A geophysical approach

Palaeogeography, Palaeoclimatology, Palaeoecology 535 (2019) 109345

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Ordovician carbonate mud mounds of the Baltoscandian Basin in time and space – A geophysical approach

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T.C. Levendala, , O. Lehnertb,c,d, D. Sophera, M. Erlströme,f, C. Juhlina a

Department of Earth Sciences, Uppsala University, Villavägen 16, SE-75236 Uppsala, Sweden Key Laboratory of Economic Stratigraphy and Palaeogeography, Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences, Nanjing, China c GeoZentrum Nordbayern, Lithosphere Dynamics, Friedrich-Alexander University of Erlangen-Nürnberg, Schloßgarten 5, D-91054 Erlangen, Germany d Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Praha 6 - Suchdol, Czech Republic e Geological Survey of Sweden (SGU), Kiliansgatan 10, 223 50 Lund, Sweden f Department of Geology, Lund University, Sölvegatan 12, SE-223 62 Lund, Sweden b

A R T I C LE I N FO

A B S T R A C T

Keywords: Baltica Sweden Gotland Carbonate mud mounds OPAB dataset Seismic interpretation

Carbonate mud mounds developed within the Baltoscandian Basin, an epicratonic basin on the Baltica palaeocontinent, during the Ordovician. In the Upper Ordovician succession of the Baltoscandian Basin, a large number of mud mounds are present at three stratigraphic levels namely, the Kullsbergs mounds (late Sandbianearly Katian), Nabala and Rakvere mounds (middle Katian), and the Boda mounds (late Katian). These formed in a subtropical-tropical carbonate platform environment, covered by a shallow epicontinental sea. The mud mounds at these stratigraphic levels beneath and around Gotland have been characterized using a comprehensive seismic and well dataset acquired during a period of hydrocarbon exploration and exploitation which began more than 30 years ago. Interpretation of the largely unpublished seismic data in this study provides details on the distribution of mound complexes in the basin and constraints on the geometry of the mounds. Detailed structure contour maps of the top and the base of the Ordovician succession beneath Gotland based on the seismic interpretation are presented. The results give a comprehensive characterization of carbonate mud mound generation on Gotland which may help in understanding the distribution patterns of similar mound complexes in other parts of the Ordovician world formed in similar environments.

1. Introduction Carbonate mud mounds have for several decades been widely considered as typical Palaeozoic structures, with their peak occurrence during favourable icehouse conditions (Krause et al., 2004) in the Late Devonian (e.g., Belgium, Boulvain, 2007 with references therein) and Early Carboniferous (e.g., Waulsortian mounds, Bridges et al., 1995, Gutteridge, 1995, Lees and Miller, 1995). These carbonate mud mounds formed as carbonate mud-dominated deposits containing stromatactis, which represents one typical type of cavity fill with marine cements (Bathurst, 1980). Stromatactis, widespread in the Katian Boda mounds in Dalarna and the Klasen mounds on Gotland, was described for the first time based on observations from Belgian mud mounds by Dupont in the year 1881. Dupont (1881). However, as pointed out by Krause et al. (2004), fenestral fabric must be recognized as just one of several typical features in such carbonate buildups. Since the 1990s the concept of carbonate muds mounds containing stromatactis has widened and the geologic history of carbonate mud ⁎

mounds has been investigated in some detail. They range from Proterozoic to recent times, thus spanning about one billion years. The oldest mounds in Earth's history are the up to 3 km wide and 100 m high pinnacle-shaped mounds in the Late Neoproterozoic Nama Group of Namibia (Wood, 1999). These are in size comparable to some of the Katian Boda mounds in the Baltoscandian Basin (Kröger et al., 2016). Phanerozoic palaeogeographic reconstructions show the wide distribution of mud mounds from the tropics to subpolar areas flourishing presumably during overall cool or cold global climates (Krause et al., 2004). Many detailed studies on facies, microfacies, petrography, cements and faunal content led over the last decades to a clearer understanding of mound formation and characteristics to distinguish carbonate mud mounds from reef mounds and reefs (see compilations in Reitner and Neuweiler, 1995; Reitner et al., 1995; Monty et al., 1995; Riding, 2002; Flügel, 2004; Reitner and Thiel, 2011, with references therein). Schlager (2003) suggested the ‘mud-mound factory’ as one of the main carbonate production systems besides the tropical ‘shoal-water factory’

Corresponding author. E-mail address: [email protected] (T.C. Levendal).

https://doi.org/10.1016/j.palaeo.2019.109345 Received 4 July 2018; Received in revised form 24 August 2019; Accepted 24 August 2019 Available online 29 August 2019 0031-0182/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Late Ordovician palaeogeography showing the subtropical and tropical position of Baltica during Katian times at about 450 Ma with the regions with mud mounds and mound complexes compiled and referenced by Webby (2002), Pas et al. (2009) and Harper et al. (2014). The palaeogeographic map represents a part of the reconstruction (Mollewide projection) provided with permission from Colorado Plateau Geosystems Inc. © 2000.The file was previously available on the website of Prof. D. Blakey (http://www2.nau.edu/rcb7/). The newest maps are © 2016 and can be found on their website http://deeptimemaps.com. The Gotland study area (Klasen mounds) is marked by a red dot. Some of the other locations (green dots) may represent more than one area with mound formation, SP marks the South Pole. (1–3) Laurentia. (1) Mackenzie Mountains, NW Canada. (2) North Greenland. (3) Anticosti Island, E Canada. (4–8) Baltica. (4) Novaya Zemlya, Russian Arctic. (5) Pechora Urals, Russia. (6) Oslo district, Norway. (7) Siljan area, Sweden (Kullsberg and Boda mounds). (8) Eastern Baltic (Estonia and Latvia, Pirgu mounds). (9, 10) Avalonia. (9) Ireland (Kildare and Portrane Limestones). (10) England (Keisley Limestone). (11) Kazakhstan. (12, 13) subtropical peri-Gondwana. (12) South China Plate (eastern China, Sanjushan Fm). (13) India (Godavari mounds). (14, 15) subpolar peri-Gondwana. (14) Spain (Cystoid Limestones). (15) Morocco (Bryozoan mounds). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

and the ‘cool-water factory’, depending on the corresponding type of sedimentation, environment and precipitation. Abiotic and biotically produced precipitates play a dominant role in this factory (Schlager, 2003). Not so much attention was drawn on the Ordovician record in the past, even though mud mound formation was known from different intervals and palaeolatitudes (e.g., Webby, 2002; Pas et al., 2009; Harper et al., 2014). Carbonate mud mounds formed preferentially during warmer intervals in icehouse periods, such as during the Katian Boda event (Fortey and Cocks, 2003) (Fig. 1). Where preserved, the mud-mounds, like huge parts of the shelf areas, were karstified during sea-level lowstands and subaerial exposure. This is observed in the Baltoscandian Basin in the case of the late Sandian and early Katian Kullsberg mounds (Calner et al., 2010a), as well as in the case of the Katian Boda mounds (Kröger et al., 2016). The sedimentary record in the Baltoscandian Basin provides not only a great possibility to study sea-level changes, shifts in palaeoclimate, changes in palaeo-environments and faunal communities in detail, but also to investigate the establishment of reef ecosystems and the formation of mud mounds in different settings on the shelf. Ordovician carbonate mud mounds of the Baltoscandian Basin are not as well characterized as the Silurian barrier reefs on Gotland, Sweden and Saarema, Estonia. In the literature, the appearance of Ordovician mound complexes, reef mounds and reefs in the epicratonic basins of Baltica are often linked to the northwards drift of this palaeocontinent from circumpolar and high latitudes in the Early Ordovician (cold, cool and temperate environments) into low latitudes close to the palaeoequator in Late Ordovician times (e.g. Jaanusson, 1973, Webby, 1984, 2002, Cocks and Torsvik, 2005, see Kröger et al., 2017 with references

therein). Climate and corresponding sea-level changes were controlling factors for mound formation and reef growth, and mud mound complexes formed in periods of high sea levels during the late Middle and Late Ordovician glacials (Kröger et al., 2016). Reefs and carbonate mud mounds have a wide distribution within the previously more extensive epicontinental sedimentary cover of Baltica (Fig. 2) and contributed in some areas significantly to the carbonate production within the Ordovician (see the compilation of Ordovician reef distribution by Kröger et al., 2017). A significant part of this Palaeozoic cover has been carved out mainly by Cenozoic river erosion, i.e., north of the present Baltic Klint (“Baltic Klint Complex”, Tuuling and Flodén, 2016, Tuuling, 2017; Fig. 2), a 1200 km long escarpment, overprinted by Quaternary glacial erosion and generally representing the northern limit of the Ordovician carbonate rocks on the Fennoscandian Shield. However, in a few small outliers north and west of the Klint (such as the Siljan area, central Sweden), reefs and mounds are present (Kröger et al., 2017), which suggests that large areas existed where such structures had formed and are now missing due to erosion. During Katian times, extremely large reef complexes developed along the palaeo-northern shelf edge in the Timan–Pechora Basin (e.g. Antoshkina, 1996, 1998). This peri-cratonic basin is currently thought to represent the eastern margin of Baltica and was located close to the palaeo-equator during the Late Ordovician (Cocks and Torsvik, 2005). The palaeoclimate during the Late Ordovician and Early Silurian was assumedly noted as a cooler period. Thus, after continuous climate cooling (e.g., Trotter et al., 2008) the growth of microbial buildups declined (Webby, 2002; Henriet et al., 2014). However, in the Late Ordovician and Early Silurian tropical and subtropical zones (Baltica, Laurentia and other palaeocontinents) carbonate mud mound 2

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Fig. 2. Occurrences and regions of Late Ordovician carbonate mound development in the Baltoscandian Basin known from outcrop and subsurface data (green circles: 1 - Ullerntangen, Oslo District, Norway (Hanken and Owen, 1982); 2 – Siljan Ring, Dalarna, Sweden (Kröger et al., 2016; Kröger et al., 2017, with references therein); 3 – subsurface Bothnian Sea (for references see Kröger et al., 2017); 4 – subsurface Östergötland (Jaanusson, 1979); 5 – subsurface east of Öland (Flodén, 1980); 6 – offshore Latvia (e.g., Kanev et al., 2001); 7 – Eastern Baltic (subsurface and outcrop data, numerous sources, see Kröger et al., 2017); the Gotland study area is marked by a red circle) plotted on a palaeogeographic reconstruction modified from Nielsen (1995) and Stouge (2004). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

The Upper Ordovician Boda mounds in the eastern part of the Siljan Ring are Katian in age (Ebbestad and Högström, 2007, with references therein) and have been studied recently in great detail by Kröger et al. (2016), but there are also smaller mound complexes in deeper water settings in the subsurface of SE Latvia (Ulst, 1972; Ulst et al., 1982). In the eastern part of the Siljan area, the early Katian Guttenberg isotope carbon excursion starts in the lowermost Kullsberg Limestone and the formation of the Kullsberg mounds terminates during the peak interval of the globally well documented GICE (Guttenberg Carbon Isotope Excursion; Calner et al., 2010a), a timing which may be compared to the reef formation in the Tuula area in Estonia (Kröger et al., 2014). To date most work on carbonate mud mounds within the Baltic Basin is based on observations in wells and outcrops. Middle Ordovician carbonate mud mounds are well exposed in the St. Petersburg area of Russia (Federov, 2003; Kröger et al., 2017) and Upper Ordovician carbonate mud mounds can be studied in outcrops in the Ordovician successions of western Estonia (e.g., Vormsi Island) and in the Siljan area of central Sweden. The latter buildups are also identified in numerous drill cores from the Estonian and Swedish mainlands, from Gotland, in western Latvia and central Lithuania, as well as from the Estonian islands of Saaremaa and Hiiumaa (Tuuling and Flodén, 2000). The dataset used for this study has recently become available and consists of largely uninterpreted, unpublished seismic and well log data.

complexes with a high percentage of algae and microbially produced lime muds developed (Harper et al., 2014), especially during a period of global warming and high sea level termed the Boda event by Fortey and Cocks (2003). Like in other areas, and based on regional outcrop and core observations, many of the Baltic carbonate mud mound complexes represent mud mounds with some bafflers and stromatolitic structures rather than reef mounds or ‘real’ reefs (e.g. Kröger et al., 2016). The mounds display a range of different sizes depending on the water depths and palaeogeographic position on the shelf. The Late Ordovician Kullsberg and Boda mound complexes formed in distal and deeper environments of the overall shallow epicontinental Baltoscandian (Kröger et al., 2016; Kröger and Aubrechtová, 2019). Unlike many other Phanerozoic mud mound occurrences, the Upper Ordovician carbonate mounds offshore Lithuania are the main petroleum reservoir rocks (Kanev et al., 2001). Seismic surveys over similar mounds in the central Baltic Sea, between Gotland and Estonia, indicate that the sizes of the mound structures are dependent on the palaeodepth of the platform (e.g. Tuuling and Flodén, 2007). Many of the carbonate buildups of the Upper Ordovician on the Baltoscandian shelf (see Fig. 2, areas of major mound formation) represent carbonate mud mounds rather than real reefs and their stratigraphic record reflects some diachronism if the occurrences are compared with respect to their stable isotope record (Kröger et al., 2017). 3

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Fig. 3. Stratigraphic subdivision, lithology, gamma ray log and synthetic seismogram of the Ordovician succession in the Grötlingbo-1 well. The location can be found on the insert map. The projection used for the map is RT90 2.5 GONV. The global series and stages are based on the classification by Bergström et al. (2009). Two synthetic seismograms are shown, one for a 60 Hz and one for 120 Hz Ricker wavelet, demonstrating the typical seismic response for the Ordovician interval for the lower resolution Vibroseis data (south) and higher resolution Mini-soise data, respectively.

Lodgepole mounds) display mound thicknesses of 40–100 m in the subsurface and dimensions ranging from an area of 0.8 × 1.6 km to 5.6 × 8.8 km (Kupecz et al., 1996). Another example for seismically studied Waulsortian mound complexes is the Dublin Basin of Ireland (De Morton et al., 2015), an area where Lees and Miller (1995) already estimated the existence of thousands of mounds in the Mississippian succession. There are detailed seismic records of Neogene (mid Miocene-Pliocene; McDonnell and Shannon, 2001) mound complexes in the Porcupine Basin (Hovland, Magellan, and Belgica mound complexes; e.g., Shannon et al., 2001, Henriet et al., 2001, Bailey et al., 2003, Van Rooij et al., 2003, Huvenne et al., 2007, with references therein). Here, mound dimensions are often comparable to the sizes of the Upper Ordovician Boda mounds in the Baltoscandian Basin, some Belgica and Magellan mounds in that region reach maximum heights of about 600 m and maximum diameters of about 3 km (Naeth, 2004). Specifically, the aims of this study across parts of the Baltoscandian Basin are: (1) to construct detailed structure contour maps of the top and base of the Ordovician beneath Gotland; (2) to map the location of the carbonate mounds beneath Gotland; (3) to characterize the mound geometry (width and thickness); (4) to provide a useful case study for comparison and evaluation of coeval mound complexes forming during sea-level highstands in the Late Ordovician; (5) to relate the OPAB stratigraphic age of mounds found on Gotland (within the Klasen limestone formation) to the stratigraphy of the Swedish Ordovician (Katian); (6) to distinguish any changes in mound geometry related to the palaeogeographic position on the Baltocandian shelf.

As a result, there is a prospect of utilizing seismic reflection methods to image and characterize these mounds. Previously there have been a range of studies to investigate the occurrence of carbonate mounds using offshore seismic data in the Baltic Basin (e.g., Flodén, 1980; Tuuling and Flodén, 2000, 2007; Sopher et al., 2016). Onshore, a detailed review of the Ordovician carbonate mounds based predominantly on well data was presented by Sivhed et al. (2004). To date, no extensive studies utilizing onshore seismic data have been presented. Therefore, in this study, we utilize a largely unpublished seismic and well database collected by the Swedish Oil and Gas Prospecting CO (OPAB) onshore Gotland in an attempt to map the location of carbonate mounds within the Ordovician and to gain some insight into their typical geometries. Previous studies (Flodén, 1980; Tuuling and Flodén, 2000, 2001, 2007) have shown that the limited outcrop data represent just the ‘tip of the iceberg’ with respect to the extension and volume of mound carbonates formed in such areas. In contrast, seismic data can allow a more quantitative estimate of mound occurrences and geometries. Detailed seismic studies have been performed for different time slices and in many different areas. For comparisons it may be instructive to see examples of seismic investigations dealing with large subsurface areas with densely packed mound occurrences such as the classical Waulsortian mound complexes and the Neogene mound complexes in the Porcupine Basin NW of Ireland. Lower Carboniferous Waulsortian-type mounds are known from many areas, seismic studies for petroleum exploration in the Williston Basin, USA (Mississippian 4

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2. Geological setting

Klasen limestone on the island increases from about 22 m in the south to about 74 m in the north (Fig. 4). The overall Ordovician sequence beneath Gotland generally increases in thickness towards the northwest and can range between 100 m–150 m (Unpublished OPAB reports 1970–1990, Sivhed et al., 2004). According to Tuuling and Flodén (2007, 2009), stratigraphy and thickness are highly variable especially in the uppermost Ordovician and lowermost Silurian. This is mostly due to Cenozoic river erosion and affects the seismic signature variations between the southern and northern Gotland. Besides the OPAB subdivision there is an older one in the Grötlingbo-1 core by Kjellström (1971) based on studies of the microplankton (Baltisphaerids). In this core, the succession across the Ordovician–Silurian boundary was subdivided in detail by conodonts in a study by Männik et al. (2015). Their work indicates that the Silurian–Ordovician boundary is at a deeper level in the core than according to the OPAB definition, which is based on the major shift from marlstone to limestone and shale (Fig. 4) Therefore, the top of the Ordovician is slightly deeper than noted in the OPAB well reports (Erlström and Sopher, 2019). The stratigraphy of the Upper Ordovician mounds was investigated by Bergström et al. (2004) and Kröger et al. (2017). Both publications verify that the main phase of mound development on Gotland occurs in the Katian. The mound-bearing seismic interval, including seismic markers O4–5 and S1 (between the Upper Ordovician Fjäcka Shale and the base of the Silurian) as defined by Tuuling and Flodén (2000), correlates well with the File Formation defined by Bergström et al. (2004) (Fig. 3), and also with the Klasen limestone in the OPAB stratigraphy. The Ordovician limestone succession is covered by a several hundred meters thick Silurian marlstone and limestone succession, which constitutes the bedrock surface on Gotland (Calner et al., 2005).

2.1. Platform cover and its stratigraphic subdivision around Gotland The Palaeozoic sedimentary succession in Sweden and the western part of the Baltic countries formed in the epicratonic Baltoscandian Basin. The basin is confined to the northwest by the highlands of the Scandinavian Caledonides and is located on the East European craton (Harff and Björk, 2011). The recent Baltic Basin, centered on the south Baltic Sea, is predominantly a synclinal structure in the area of the regionally extensive Early Palaeozoic Baltoscandian Basin. Deposition within the Baltoscandian Basin began in the late Proterozoic/earliest Palaeozoic during the breakup of the Rodinia supercontinent and the opening of the Tornquist Sea (Poprawa et al., 1999). In this chapter, especially the island Gotland, in the northern part of the epicratonic Baltoscandian Basin as well as adjacent study areas are discussed. Despite of stratigraphical subdivisions of the Ordovician strata by Kjellström (1971) and Männik et al. (2015), we used the traditional OPAB subdivision in this seismic study for the Cambrian and the Ordovician successions. This decision is based on the fact that the OPAB subdivision of the Palaeozoic sequence relies largely on petrophysical properties which can be more easily correlated with the seismic data. It is easier to relate the OPAB well site descriptions to their stratigraphic definitions. Otherwise a more actual division of the Cambrian is presented by Nielsen and Schovsbo (2007) and a more refined definition of the whole Ordovician succession beneath Gotland is presented by Erlström and Sopher (2019). Fig. 3 shows how the informal OPAB stratigraphy can be correlated with the regional Ordovician lithostratigraphic scheme and to the stratigraphic subdivisions by Kjellström (1971) and Männik et al. (2015). Formations and lithologies mentioned in this chapter have been identified from rock samples captured during drilling, cores and wireline logs interpreted mainly from the OPAB well reports together with the exploration reports dated to 1976. The sedimentary sequence of Gotland is resting on the Precambrian crystalline basement and composed of Lower Palaeozoic siliciclastic (Cambrian through lowermost Ordovician) and calcareous (Lower Ordovician and Silurian) strata. The Precambrian Basement beneath Gotland typically consists of highly weathered biotitic-gneisses (OPAB unpublished reports, 1976). The overlying Cambrian sequence, as defined by OPAB, is represented by five formations (Fm) namely, the Viklau, När, Tessini, Faludden and Alum Shale formations. The Viklau and När formations consist of alternating beds of sandstone, siltstone and shale. The Tessini Sandstone is dominated by shale with thin interbeds of sandstone and siltstone. The Faludden Sandstone is characterized as a fine to medium grained sandstone unit and the Alum Shale Fm as a condensed sequence of mainly black shales (Fig. 4). The lithostratigraphic subdivision of the Ordovician limestone succession beneath Gotland is not yet thoroughly constrained and the informal OPAB scheme from 1974 with three units, i.e., the Bentonitic, Kvarne and Klasen limestone units (Fig. 3), is still widely used. The Bentonitic limestone thickness on Gotland typically increases from north to south (Fig. 4), with a minimum thickness of about 28 m in the north and about 59 m in the south. This unit, composed of dense, reddish and gray microcrystalline limestone beds and highly glauconitic succession in its basal part, is often referred to as “orthoceratite limestone”. The Kvarne limestone is recognized by a heterogeneous succession of argillaceous limestone, mudstone and shale, which give relatively high natural gamma values and relatively low-density readings in the wire-line logs. The thickness of this formation generally decreases towards the southern part of the island where it is approximately 5 m thick. In contrast, the thickness in the subsurface of the northernmost parts of the island is around 11 m. The uppermost unit within the Ordovician OPAB stratigraphy is the Klasen limestone, which is dominated by a variety of argillaceous limestones and mudstones with thin shaly interbeds. The limestones in this unit are typically microcrystalline and dense. The thickness of the

2.2. Upper Ordovician mounds around Gotland - characteristics and facies Within the Upper Ordovician of the Baltoscandian Basin, carbonate mud mounds are found in three stratigraphic levels, the late Sandbian to the early Katian, the middle Katian (Nabala and Rakvere mounds) and the late Katian. Flodén (1980) noted that the “Middle Ordovician” mounds, which in modern chronostratigraphy are Sandbian to early Katian in age, appears to increase in thickness from west to east (from Sweden towards Estonia) and that the “Upper Ordovician” mounds (late Katian) rest occasionally on the earlier “Middle Ordovician” mound generation. They attain their greatest thickness and width in the offshore area to the north of Gotland, which Flodén (1980) attributed to the presence of high water current velocities at the time of deposition that controlled their growth. Flodén (1980) concluded that the central Baltic distribution of mounds must have been environmentally controlled since no erosional remnants of carbonate mud mounds are present outside the Upper Ordovician. Bergström et al. (2004) describe two generations of mounds beneath Gotland which are Katian (Late Ordovician) in age. The Liste mounds (Rakvere and Nabala stages) and the younger Klasen mounds (Pirgu Stage). The latter are part of a wide mound and reef belt extending to the northeast for about 300 km from Gotland towards Hiiumaa Island (Kröger et al., 2017). The offshore carbonate mud mounds in deeper settings are, based on seismic reflections and biostratigraphic data on cores, latest Katian (Pirgu) through Hirnantian in age (Bergström et al., 2004; Tuuling and Flodén, 2000, 2007). Carbonate mud mounds up to 2 km in diameter exist offshore close to Gotland (Flodén, 1980; Flodén et al., 1997; Tuuling and Flodén, 2000). According to Tuuling and Flodén (2000), the dimensions and outline of these Ordovician carbonate buildups have a configuration ranging from strongly irregular to more regular structures of varying geometry. Only a few mounds have been recognized by Tuuling and Flodén (2000) below their regional seismic O4–5 reflector (i.e., Fjäcka Shale level) (Fig. 3). They also suggested that carbonate buildups situated above their O3 seismic reflector (Macrourus limestone unit) 5

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N

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Fig. 4. A correlation panel of the Palaeozoic succession in a north-south transect across the island of Gotland. The lithologies in the wells are interpreted based on well reports and geophysical well logs. Vertical scale is in true vertical depth subsea (TVDSS).

from shallow water dominated environments in the North Estonian Confacies Belt to inner shelf settings in the Central Baltoscandian Confacies Belt. Deeper marine conditions prevailed to the southwest in the Scanian Confacies Belt (Fig. 2). The depositional environment in the Upper Ordovician succession on the northernmost part of Gotland was quite comparable to shallow shelf environments in northern Estonia (Eriksson and Hints, 2009). According to interpretations in the unpublished OPAB reports, the deposits in the Upper Ordovician succession can be divided into two major facies. 1) Reefal limestone facies, classified as biomicrites. 2) Non-reefal argillaceous limestone facies, which are composed of biomicrites and minor sparitic limestones and commonly contain shale beds up to 0.3 m. Sivhed et al. (2004) distinguished four different lithofacies associated with the Upper Ordovician mounds from drill cores, which would fall into the OPAB classifications of ‘reefal’ and ‘non-reefal’ limestone facies. These lithofacies are mainly based on petrological studies of core material, well reports and, geophysical logs (Sivhed et al., 2004). According to OPAB's scheme, the supra mound and submound facies, consisting of laminated argillaceous limestone and

(Fig. 3) could probably correspond to the oldest, Rakvere and Nabala stages (Liste mounds), reef-like structures, known from several places in the Baltic region (Männil, 1960; Jaanusson, 1979). Similar structures also exist further towards the west (Flodén, 1980). Sivhed et al. (2004) discovered that the mounds on Gotland have a maximum diameter and relief of 800 m and 25 m, respectively. Small faults are frequently associated with carbonate mud mounds beneath Gotland, which are probably due to differential compaction around the mound during burial (Sivhed et al., 2004). Carbonate mud mounds are found most commonly within the Upper Ordovician to the north of the basin. They occur beneath Gotland and offshore to the east of Gotland. The distribution reflects a general shallowing towards the north of the basin. Between Gotland and the southwestern part of Finland, is an area with relatively large numbers of mounds, suggests an area of relative shallow water conditions during the Ordovician. In a number of studies, this feature has been referred to as the Gotland–Gotska Sandön ridge (Männil, 1966; Jaanusson, 1975; Flodén, 1980; Kiipli et al., 2008). The Baltoscandian Basin in the Ordovician was subdivided into confacies belts (Jaanusson, 1976, Fig. 2) which reflect different bathymetric conditions and depositional settings 6

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irregular beds of dense wackestones and packstones would be classified as ‘non reefal’ limestone facies. The cap and flank facies, composed of packstones with finely disintegrated bryozoans and tabulates and the intra mound facies, which consist of algal packstones, vugs and fenestral pores would be referred to as ‘reefal’ limestone facies.

Table 1 Processing steps applied to the land seismic data from Gotland.

3. Materials and methods 3.1. Reflection seismic data The presence of an Ordovician carbonate mound typically has two effects on the gross Ordovician sequence: 1). There is a local increase in the gross thickness of the Ordovician at the mound location and 2). a local dome like structure is present on the top of the Ordovician, or sometimes within the Ordovician sequence. Such changes can typically be detected using seismic reflection data and hence, in this study we aim to map and characterize the geometry of the mounds using the seismic response. The seismic and well data acquired by OPAB during the 1970s and 80s (Sopher and Juhlin, 2013) is currently hosted at the Geological survey of Sweden (SGU). The dataset on Gotland includes information from over 300 wells and over 2000 km of 2D land seismic reflection data (Fig. 5). So far, this dataset has not been used either for studying the Ordovician carbonate mud mounds or generating structure contour maps on Gotland. A Vibroseis or mini-soise source was used to acquire the land data, typically with a relatively low fold (12). The shot point interval for each survey was every 30 m. The record length was either 1 s or 0.5 s and the sampling interval was either 1 ms or 0.5 ms. Processing of the acquired seismic reflection data resulted in a series of final stacked sections

Processing steps for survey (P)

Processing steps for survey (MS) and (MC)

Seismic source: Vibroseis Gain removal True amplitude recovery (TAR) Deconvolution Static correction Dynamic correction Horizontal stacking Frequency filter Normalized

Seismic source: Mini Soise Resampled Spherical divergence Edit gathers Static correction Deconvolution Normal move out (NMO) Mute Corrections Residual statics CDP Statics CDP stack Deconvolution Bandpass filter Scaling

which have been used for interpretation. Table 1 shows the typical processing steps applied to the data collected with both the Vibroseis (P surveys) and Mini-Soise sources (MS and MC surveys). An important consequence of the two different seismic sources utilized in the surveys is the frequency content, which affects the resolution of the data. Therefore, the older surveys using a lower frequency Vibroseis source (P) have a vertical resolution of approximately 14 m while the newer surveys using a Mini Soise source (MS and MC) have a vertical resolution of approximately 7 m. The older surveys are predominantly located in the southern and eastern parts of Gotland, resulting in poorer resolution of the mounds in these parts of the island (Fig. 6). Some seismic data within the dataset are available in digital SEGY format, but most of the data are only available as raster images (tiff format) displaying the final processed sections. The raster images were converted (vectorized) to SEGY format for obtaining a set of seismic lines on which modern seismic interpretation software can be used. The Matlab script WIGGLE2SEGY (Sopher, 2016; Sopher, 2017) was used to convert the tiff images to SEGY format.

Q

3.2. Well data and synthetic seismograms

57.68o

Using geophysical well logs (namely P-wave sonic and density logs) synthetic seismograms can be generated which indicate the theoretical response of the seismic reflection data at the location of the well. However, despite the large amount of wells drilled on Gotland, key geophysical well logs were available only from 38 boreholes. As with the seismic reflection data, the wireline logs were only available as raster images (tiff format) that were digitized using open source software. After digitization, synthetic seismograms were constructed and used to identify key seismic reflections in the data (Fig. 6).

P

57.25o

4. Results 4.1. Analysis of the synthetic seismogram

Legend Wells MC/MS Seismic Surveys P Seismic Surveys Cross section profiles

N

In the Ordovician interval, the most important regional seismic markers occur at the top and base and appear as relatively continuous positive and negative reflections, respectively (Fig. 6). These markers typically generate strong reflections because of the large acoustic impedance (the product of density and seismic velocity) contrasts between the Ordovician limestone and the sub-cropping Cambrian and the overlying Silurian successions. However, it should be noted that the strength of the top Ordovician reflection is typically less towards the north of Gotland, compared to the south. This is due to the generally higher clay content in the upper Ordovician interval to the north when compared to the south (Fig. 6) as well as the erosional Ordovician-Silurian boundary below Gotland which varies between the northern and

M 18.29o

19.12o

Fig. 5. Locations of the seismic profiles interpreted in this study. Note that wells are correlated to the seismic profiles using synthetic traces. Dotted lines represent the MC and MS seismic surveys and the solid gray lines represent the P seismic surveys. The solid black line P–Q and M–N illustrate the location of two cross sections described in Section 4.1. 7

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A

Skäggs_1

W

B E

Linhatte_1

NE

SW

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Legend TO Top Ordovician reflector BO Base Ordovician reflector

TO

BO

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Time (s)

0.3

Time (s)

0.1

TO BO 0.4

Gotland

B

0.5 300

400

500

A

300

400

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CDP

CDP

Fig. 6. Seismic profiles are plotted time (s) against the common depth point (CDP). Seismic lines A, MS77-209 and B, P75-4 acquired using a mini-soise source and a Vibroseis source, respectively. Synthetic seismograms shown in both diagrams are represented by A, Skäggs-1 well, located in the north of Gotland and B, Linhatter-1 well, located in the south of Gotland. The dotted line (red) correlates key seismic events with the well data within the Ordovician. The inserted map indicated the locations of seismic lines A and B. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

within the Ordovician interval, which were also interpreted to indicate mounds. Concave upwards reflections within the Ordovician such as this, typically only occur in the northern part of the island (Fig. 7B). We suggest this to be due to that the upper part of the Ordovician sequence is more continuous in the south of the island as mentioned before, and the strata overlaying the carbonate mounds are gradually changing and more clay rich than in to north where significant gaps in the Late Ordovician sequence and at the Ordovician/Silurian boundary may be assumed. This could lead in the south to a greater impedance contrast between the overlying uppermost Ordovician sequence (mixed limestone and shale) and the underlying carbonate mound (predominantly limestone) and therefore give a stronger seismic response at the top of the mound. In practice, a local horizon was interpreted in the seismic data only at the mound location. This horizon was picked to follow the uppermost anticline reflection in the data within the Ordovician interval (red solid lines in Fig. 7). We were unable to interpret the base of the mounds in the seismic data due to the lack of a related consistent reflection. This is presumably caused by the relatively similar P-wave and density values between the base of the mound and the underlying Ordovician sequence. Two cross sections, M–N and P–Q (Fig. 5) located in the southern and northern part of Gotland have been interpreted and are shown in Fig. 7A and B, respectively. In these sections the top of the Precambrian basement has been tentatively interpreted as a weak positive amplitude reflection. A positive reflection for the top basement is likely as the seismic velocity and density probably increase in the transition from Cambrian siliclastics to a basement dominantly composed of gneisses. The top of the basement in general, appears as a relatively smooth structure on the lines shown here (Fig. 7A, B), which is consistent with a period of prolonged erosion before the onset of deposition during the Cambrian, i.e. the Precambrian peneplain (Flodén, 1980). No key reflections have been interpreted within the Cambrian interval on profiles M–N and P–Q however, according to OPAB's well data, the thicknesses of the respective stratigraphic formations within the Cambrian interval remain relatively constant in these areas. Within seismic profiles M–N and P–Q, a series of weak semi-parallel reflections are observed, consistent with the constant thicknesses observed in the well data.

southern part of the island. Within the Ordovician interval, the argillaceous Kvarne limestone unit (Fig. 4) provides a sharp velocity and density contrast with respect to the overlying and underlying lithostratigraphic units. However, the thickness of the argillaceous beds in the Kvarne unit are often very thin, therefore they cannot be distinguished on the older Vibroseis (P) surveys with lower resolution in the southern parts of the island (for example in Fig. 6). In some parts of the island, the rather thin Kvarne limestone interval can still produce an additional reflection in the more recent, higher resolution, Mini Soise surveys (MS and MC). This occasional reflection, however, is challenging to interpret across the whole island and was not considered in this study.

4.2. Seismic interpretation Utilizing the synthetic seismograms, the seismic reflections from the top and the base Ordovician were interpreted on all seismic profiles available on Gotland, as relatively continuous positive and negative reflections, respectively (Fig. 7). Fig. 7 exemplifies the characteristics of the high resolution seismic data (Mini Soise) in the northern part of the island and the lower resolution seismic data from the southern part of the island (Vibroseis source). Efforts were also made to interpret mounds within the seismic data. Typically, local anticlinal or dome-like structures on the top Ordovician reflection were considered as indications of mounds. We assume that these structures on the top of the Ordovician interval are erosional remnants of mounds (Fig. 7A). The majority of mounds interpreted in the southern part of the island appeared only as concave upwards features on the top of the Ordovician succession. The local situation in the subsurface around Gotland was recently summarized by Erlström and Sopher (2019) pointing out that based on the work of Tuuling and Flodén (2000, 2007, 2009) seismic data display erosional unconformities in the Late Ordovician succession beneath and northeast of Gotland (sea level fall during Hirnantian glaciations, intra Hirnantian, and across the Ordovician/Silurian boundary). In this context, the deeper environments in the Upper Ordovician strata offshore south of Gotland and on south Gotland only show some gradual shallowing while to the northeast of Gotland hiatuses characterize this part of the sequence (Erlström and Sopher, 2019). In some cases, similar domelike features were also distinguishable 8

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Fig. 7. Illustrated are two cross section profiles, M–N (A) and P–Q (B) with a length of 5.5 km and 28 km, located in the south and north of Gotland respectively. Refer to Fig. 5 for the location of these profiles. Both profiles consist of an interpreted ‘Precambrian basement’ as well as a ‘Cambrian’ sequence. This is overlain by an ‘Ordovician’ sequence, and above this, the ‘Silurian’. Dashed red lines represent the key horizons interpreted. The black box in both profiles, highlights the position of a carbonate mud mound. The top of the mound horizon is indicated by a solid red line. The mound has also been interpreted in the close-up box. Note that the time and distance scales are different for the two profiles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

the south of Gotland carbonate mud mounds occur less frequently and are not as closely spaced. They also appear to be smaller than in the northern part of Gotland (Sivhed et al., 2004). In this study we apply a statistical approach to our observations from the seismic interpretation to estimate typical mound thicknesses and sizes. Fig. 9 shows a schematic image of the reflections which were interpreted in the seismic data at each mound location. To estimate the width of a given mound we measured the distance w, i.e. the lateral extent of the top mound reflection as interpreted. As mentioned earlier, it was not possible to interpret a reflection at the base of the mound, therefore we used the following equation to estimate the mound thickness (t) from our interpretation:

4.3. Ordovician thickness and top structure The structure contour map of the top Ordovician was generated using seismic profiles across Gotland (Fig. 8). The interpreted horizons were smoothed and converted to depth using well data. The top Ordovician surface is typically smooth and exhibits a gradual dip to the southeast. The mound distribution is illustrated by red dots in the map (Fig. 8). Over 150 mounds have been identified in the seismic data on Gotland. The concentration of mounds appears to be higher in the eastern part of Gotland and in the south. This is consistent with observations made by Flodén (1980) and others (Sopher et al., 2016; Tuuling and Flodén, 2000; Sivhed et al., 2004) who have suggested that a shallowing of the depositional environment towards the north and east of Gotland (Gotland–Gotska Sandön ridge) during the Ordovician.

T = v (x 2 − x1)/2000

(1)

where x1 and x2 denote the minimum and maximum two way time interval in ms between the top mound reflection and base Ordovician reflection where the mound is present, respectively. An interval velocity (v) of 3200 m/s was assumed for this calculation. This velocity was obtained from the time depth functions generated from the well tops to match the key seismic reflection of the Ordovician. At this stage we attempted to account for a factor which could significantly bias the results. The problem is that when we observe a mound on a 2D seismic line we do not know if the profile passes through the center of the mound. If we assume a dome-like geometry for a mound then the observed width and thickness will vary greatly depending upon the location of the transect over the mound (Fig. 9). Therefore, for every one of our seismic observations, we cannot guarantee that we have observed the true width or thickness of the mound. To account for this, we first plotted histograms of mound widths

4.4. Mound thickness and size To date, carbonate mud mound thicknesses and sizes on Gotland have been estimated mainly through analysis of well data. In this section we utilize our seismic interpretation of the Upper Ordovician carbonate mud mounds on Gotland to quantitatively assess the typical thicknesses and sizes of the mounds. Tuuling and Flodén (2000) used high resolution seismic data to map Ordovician mounds offshore to the northeast of Gotland. They observed a gradual decrease in mound diameter from the north (around 2 km) to the south (around 0.6 km). Based on well data and some limited seismic data, Sivhed et al. (2004) observed that carbonate mud mounds in northern Gotland have a diameter of c. 0.5 km and the distance between individual mounds in clusters have a separation of about 1–1.5 km. In 9

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Fig. 8. The main map shows the depth (below sea level) to the top of the Ordovician succession. The contour interval is 25 m. The insert shows the Ordovician thickness map in meters. The thickness ranges from 50 m to 120 m.

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Fig. 9. A). Schematic illustration showing the key seismic reflections interpreted at the location of a mound in this study. The top mound reflection and mound itself are shown as a thick dotted line and gray area, respectively. The key measurements used to calculate the mound width and thickness are annotated. B). Illustrates a map view image of a circular lenticular mound with the location of three 2D profiles D, E and F annotated as dashed lines. To the right of the image are the cross sections of the mound obtained from profiles D, E and F.

the depositional setting during the Ordovician. According to OPAB (1976), the Upper Ordovician rocks of Gotland, namely the Klasen limestone, were formed on a shallow, stable marine platform that was subject to gentle subsidence. According to Eriksson and Hints (2009), northern Gotland corresponds to Late Ordovician times, which are like those in the north of Estonia, indicating a shallow water to transitional shelf environment. The inferred boundaries between shallow shelf, transition zone and deep shelf environments during the Ordovician around the island of Gotland are fairly well defined. The boundaries between the different depositional environments, with respect to water depth, are based on Kiipli et al. (2008), but have been modified to fit with the seismic observations of mounds. Based on these observations, the area north-northeast of Gotland appears to constitute an area of relatively shallow water during the Late Ordovician (Gotland-Gotska Sandön ridge; Flodén, 1980). Hence, a greater number of mounds developed in this part of the basin compared to areas to the south where the relative water depth was greater. The inferences about the depositional environment from the seismic observations in this study show good agreement with the work of others (e.g. Jaanusson, 1973; Nielsen, 1995; Stouge, 2004).

(Fig. 10A) and thicknesses (Fig. 10B) that were observed from the seismic data for all carbonate mud mounds. To estimate the true average mound width, we performed the following steps: 1) We assumed that the mounds have a circular shape and that the true distribution describing their widths is a normal distribution with a given mean and standard deviation. 2) Based on this distribution we calculated a mound width at random. 3) We then took a random transect through this mound and recorded the observed width, which will typically be less than the true width. 4) We then repeated this process (steps 2–3) 10,000,000 times to generate a distribution of observed mound widths for a given standard deviation and mean value. 5) We then repeated steps (1–4) to generate a range of distributions for a range of standard deviations and mean values. 6) We then compared the different distributions with the histogram of the widths observed from the seismic interpretation to find the best match. In principle the best fitting modelled distribution should give an estimate of the true mean and standard deviation value for the carbonate mud mounds, hence removing the biasing effect caused by the 2D nature of the observations. Fig. 10A shows the modelled distribution which gave the best fit to the observed values as well as the mean value used to generate the modelled distribution. Based on this attempt to de-bias the results, the true mound widths have a mean of 640 m and a standard deviation of 160 m. To estimate the true average mound thickness from the seismic data we performed the same set of steps (above), however, instead of width, we assumed a standard deviation and mean value for the mound thickness. We also assumed that the modelled mound thickness varies as a cosine function with respect to the radius. Fig. 10B shows the modelled distribution which gave the best fit to the observed values as well as the mean value used to generate the modelled distribution. Here we estimate the true mound thickness to have a mean of 19 m and a standard deviation of 5.1 m. Note that the true mean mound thickness is quite different to the peak value in the thickness histogram, highlighting the importance of performing this de-biasing step. In addition to the seismic observations we also calculated a distribution of mound thickness observations made from 226 wells on Gotland from the OPAB database, which can be seen in Fig. 10C. The mean value obtained from the well database agrees well with the average value obtained from the seismic interpretation.

6. Discussion The results from this study provide valuable information about the width, thickness and location of the carbonate mounds on Gotland, however, it is important to highlight several key factors and limitations before considering the results further. One such limitation is that due to the resolution in this dataset it was not always possible to identify the mounds based on seismo-stratigraphic relationships (e.g. onlap). Instead we typically had to interpret mounds based on their structural characteristics (i.e. structural highs or dome-like features) (Fig. 7). This is a somewhat subjective means of identifying the mounds and, hence, it means that we may have falsely identified mounds or failed to identify mounds in some parts of the data. This was mainly due to the variable quality of the seismic image of the mounds and the often relatively weak and discontinuous top mound reflections. Although the average mound size from this study is similar in magnitude to those made by Flodén (1980) based on offshore seismic data, the mounds we observe on Gotland are typically smaller than offshore. From well data, the thickness of these mounds is known to vary spatially due to their inherent geometry (i.e. thinning at the flanks and thickest in the center) and due to the effects of erosion. Tuuling and Flodén (2000) noted that northeast of Gotland, the size of the carbonate mounds decreases from north to south. This could genuinely be the case and reflect a lateral change in mound size. However, it should be noted that the resolution of the offshore seismic data used by Flodén (1980) was higher than the seismic data used in this study. At the flanks the mounds gradually reduce in thickness and pinch out. Therefore, it may

5. Regional depositional environment during the Ordovician In this section we combine the locations of our identified carbonate mud mounds on Gotland with those in the offshore area around the island of Gotland to make broad inferences about the depositional setting during their formation in the Late Ordovician (Fig. 11). The location of mounds identified using offshore seismic data by others are annotated, as well as the mounds identified in this study. We propose that the location and frequency of the mounds indicate in a broad sense 11

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Fig. 10. A) Histogram of observed and modelled mound widths in meters. B) Represents a histogram of observed and modelled mound thicknesses in meters. C) Mound thickness according to OPAB dataset.

have been possible for Flodén (1980) to image the flanks of the mound in more detail and hence measure the extent of the mounds more accurately. This effect could lead to a systematic difference in the estimates of mound size between the two studies. Our study shows that the special mound ecosystems in the Late Ordovician persisted not only for millions of years (Kröger et al., 2016), but also had a widespread and dense distribution across the epicratonic Baltoscandian Basin. Future quantitative evaluations of the global occurrences of mound complexes compiled by Harper et al. (2014) vs. the skeletal reef structures during the Late Ordovician and Early Silurian times should change the view of Henriet et al. (2014, Fig. 4) with respect to a low record of carbonate mud mound formation during this interval. Mud mounds may represent large volumes in the carbonate sequences on some palaeoplates, at least during Katian times. Krause et al. (2004) suggested, based on a minimum estimate of 2308 Waulksortian mounds in the British Midland basins, that Tournaisian and Visean successions may contain more than 74% of the global Phanerozoic mud mounds. Our results referring to a limited area of the ancient distribution of mud mound complexes, however, may suggest that for other geological ages, such as the Late Ordovician, it may be just a matter of detailed seismic studies, interpretations and quantifications of mud mound numbers to demonstrate a more significant record and distribution of carbonate mud mounds. 7. Conclusions In this study we have utilized a large and predominantly unpublished seismic and well database to characterize the Ordovician strata and carbonate mud mounds beneath the island of Gotland. The Ordovician interval on Gotland dips gently to the southeast, which is related to its position on the north-western flank of the Baltic Basin. Mounds are found to occur more frequently towards the north and east beneath Gotland, supporting the idea of a structural high running along the eastern edge of Gotland during the Ordovician (i.e. the Gotland–Gotska Sandön ridge). Our main results can be summarized as following, we present for the first time detailed structure contour maps of the top and base Ordovician sequence beneath Gotland. The first map of the location of carbonate mounds beneath Gotland, based on interpretation of the seismic data is compiled and presented. The seismic observations and interpretations give a mean mound width and thickness of 640 m and 19 m, respectively and finally, this study highlights the use of the seismic reflection method as a powerful tool to characterize large scale structures like mound complexes on local and regional scales. The vast occurrences of Late Ordovician mound complexes in the Baltoscandian Basin show that they formed an extended “mound belt” on the shelf extending from the Oslo Region in Norway through the Siljan area and Gotland areas in Sweden and continuing into the Eastern Baltic area. The mound limestones constitute large volumina of the carbonate production in these regions and represent characteristic features of Katian (Late Ordovician) shelf successions, even when they are only partly preserved due to later erosion. The extensive mud mound formation on the Baltoscandian Shelf is strongly linked to global climate and eustatic sea-level changes and coincides with Late Ordovician mound development in many parts of the world. Acknowledgements We thank the Geological Survey of Sweden (SGU) for their input and providing data and the two reviewers of our manuscript for all their useful suggestions. 12

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Fig. 11. Regional map showing the typical depositional environment during the Ordovician inferred from the location of mounds (modified from Kiipli et al., 2008). Purple circles indicate the mounds identified in this study. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

T. L. is supported by the National Research Foundation, South Africa (NRF). D. S. was partially supported by the Swedish Research Council. O. L. is very grateful for the support of his research in Baltoscandia by the Deutsche Forschungsgemeinschaft (DFG project LE 867/8-1 and 82) and in the frame of project PUT 378 financed by the Estonian Research Council focusing on Late Ordovician climate and sea-level changes. He worked on the manuscript during his time as a visiting professor at Nanjing Institute of Geology and Palaeontology (NIGPAS, China). This paper is a contribution to IGCP project 653 ‘The onset of the Great Ordovician Biodiversification Event’.

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