Markes E. Johnson

Relationship of Silurian sea-level fluctuations to oceanic episodes and events

Abstract Eight distinct highstands in sea level emerged from the authors study of the Silurian System on a continent-to-continent basis during the 1980s and 1990s. Some emendations are needed to accommodate other trends recognized since that time, but the overall pattern is strong. Five major highstands and one minor highstand fit the Lower Silurian (Llandovery and Wenlock series). Three major highstands fit the Upper Silurian (Ludlow and Přídolí series). Three separate episodes of glaciation correspond to drops in sea level from the Lower Silurian but fluctuations recorded in the Upper Silurian have no clear cause. Many oceanic events share common trends when the relationship with sea-level fluctuations is tested against a tight biostratigraphic framework. The Sandvika Event correlates with a sea-level drop between the second and third highstands from the Llandovery Series. An unnamed secundo-primo event overlaps the fall between the third and fourth highstands. The Ireviken Event corresponds to a drop near the basal Wenlock. The Boge Event matches a minor draw down superimposed on the fifth rise from the Sheinwoodian Stage. The Mulde Event partly fits a drop in the Lower Homerian Stage. The Linde Event corresponds to the low between the sixth and seventh peaks from the Ludlow Series and the Klev Event fits the lowstand between the seventh and eighth peaks near the Ludlow-Přídolí boundary. The Lau Event, however, appears to adjoin the seventh highstand. No known event corresponds to the draw down between the first and second sea-level peaks recorded in the Llandovery Series.

Why Are Ancient Rocky Shores so Uncommon

The geologic history of rocky shore biotas has been neglected by paleontologists. Under present conditions of high continentality, rocky shores account for more than 33% of the worlds coastline. They are generally located on shores facing narrow continental shelves, where the pattern of global plate tectonics exerts much influence over their regional concentration. The highest frequency of rocky shores occurs on islands associated with hot spots, island arcs, and convergent-plate margins associated with subduction. Rocky shores situated in these settings are subject to recycling, so their remains are rare in the stratigraphic record. Some passive continental margins, including those along newly rifted oceans and those with low rates of sedimentation, exhibit frequencies of rocky shores as high or higher than those found on many convergent-plate margins. These have a good chance of being retained in the stratigraphic record for geologically significant periods of time. Epicontinental flooding reduces the overall extent of rocky shores during major high stands in sea level, but unconformable surfaces produced by the fluctuation between marginal seas and epicontinental seas are a common feature in the stratigraphic record. These surfaces are the only stable archives of rocky-shore biotas. Ancient rocky shores are rarely described in the paleontological and geological literature because unconformable surfaces are seldom excavated in three-dimensional relief. Important data are easy to overlook in cross-sectional slices of limited extent.

Silurian continental distributions, paleogeography, climatology, and biogeography

Abstract Continental orientations during the Silurian Period have been determined using paleoclimatic in addition to paleomagnetic data. The influence of climate on lithology is particularly marked during periods like the Silurian when epeiric seas were widespread and sedimentation was dominantly autochthonous (evaporites, carbonates, reefs, authigenic minerals) and therefore reflective of climate at the depositional site. During such times, with few large land areas in low latitudes, one would expect climatic patterns to have been more zonal than cellular, and also that long river systems (capable of transporting clastic sediments from wet to dry belts) would not have existed. Therefore, even allochthonous deposits, particularly thick sequences of coarse elastics can be added to the list of paleoclimatic indicators. Silurian northern hemisphere atmospheric circulation can be modeled on present patterns in the southern hemisphere because of the lack of significant land influence on climate. The wet-hot (10°N—10°S), dry-warm (10°—30°), wet-cool (30°-60°), dry-cold (60°—pole) pattern is amply confirmed by Silurian sediment distribution on those paleocontinents whose orientations have been established from paleomagnetic measurements (Laurentia, Baltica, Siberia). Paleozoic sedimentation in these zones is as follows: 10°N—10°S, thick elastics and reefs associated with leading plate margins, and carbonates and reefs in the epeiric seas; 10°—30°, evaporites, carbonates and reefs; 30°—60°, clastics, coals and tillites; 60°—pole mostly tillites. The other paleo-continents (Kazakhstania, North China, South China, Gondwana) can be oriented by using the above lithologic associations in ways consistent with their known convergent and collision patterns in the late Paleozoic. All were in relatively low latitudes with the exception of Gondwana which was over the South Pole. A large north polar ocean existed which must have had an ameliorating effect on climate in the northern hemisphere, while the opposite was true of the southern hemisphere. The conclusion that most paleocontinents had extensive epeiric seas and were positioned in low latitudes accounts for the cosmopolitan nature of Silurian faunas. Only Gondwana in the south ( Clarkeia fauna), and Mongolia in the north ( Tuvaella fauna) shows signs of provincialism and low faunal diversities. This situation can be contrasted with the Devonian, when the collision of Laurentia and Baltica resulted in land barriers and marked provincialism.

Intercontinental correlation by sea-level events in the Early Silurian of North America and China (Yangtze platform)

Potential for eustatic sea-level changes on continental platforms during Early Silurian (Llandovery) time was highly propitious due to the double effects of lingering glaciation in Gondwana and the rapid rate of sea-floor spreading which assisted the general dispersion of many era tonic realms through middle Paleozoic oceans. Following the initial mass melting of the Late Ordovician/Early Silurian glaciers in North Africa and Arabia, four smaller cycles of sea-level fluctuation were recorded by recurrent, shelly communities in a predominantly carbonate setting on the North American Platform. Peaks in sea-level rise occurred in late Rhuddanian or early Idwian (A 4 –B 1 positions), early Fronian (C 1 position), early Telychian (C 4 position), and late Telychian (C 6 position) times. Although absolute time control is poor, the periodicity of these events may have been every 2.5 m.y. In the more submerged regions of the North American Platform, such as Iowa and Anticosti Island, diverse faunal associations distinguished by the presence of stricklandiid brachiopods were especially productive at peak times of transgression. Co-occurrence of the calcareous green alga Cyclocrinites with certain of these faunas indicates that maximum water depth was still within the photic zone. At the other extreme, low stands in sea level are identified by diminished faunal diversity, as well as a combination of typical sedimentary structures and evaporite indicators. Located on a well-defined Precambrian craton, the Yangtze Platform is now a large region in central and southwestern China. During successive stages of the Early Silurian, a thick sequence of graptolitic shales, shelly carbonates, and red beds were deposited on the platform. Of the four sea-level peaks defined in North America, three are recognized in the Yangtze Region. There is very good evidence for a latest Llandovery or earliest Wenlock transgression, based on the extensive distribution of the Salopinella - Coronocephalus - Sichuanoceras fauna. Another clear and well-dated sea-level peak involves the occurrence of faunas with characteristic stricklandiid and pentamerid brachiopods at a level equivalent to the lower Telychian (C 4 position). The initial Silurian transgression appears to have reached its peak on the Yangtze Platform primarily during early Idwian time (B 1 position), but this is based less surely on changing patterns in graptolite diversity. At present, there is no compelling evidence for a widespread, early Fronian (C 1 position) transgression in China. Successful matching of the other sea-level events, however, substantially refines intercontinental correlation for Lower Silurian strata in the Yangtze Region of China and the central regions of North America.

Eustatic sea-level patterns from the Lower Silurian (Llandovery Series) of southern Norway and Estonia

This study demonstrates the utility of checking for eustatic sea-level 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 Estonia9s 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 high-stands 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.

Diversification of rocky-shore biotas through geologic time

Abstract Changes in biodiversity of rocky-shore ecosystems from the early Precambrian (3,500 Ma) to the last interglacial epoch (125 Ka) are summarized on the basis of the fossil record associated with geological unconformities that reflect coastal paleotopography. This analysis is derived from data reported in 130 published papers culled and updated from previous bibliographic reviews. Minimum total diversity of fossil and extant species treated herein is 655 species. The highest biodiversity from any single locality is a mollusk-dominated biota of 62 species from San Nicolas Island on the Pacific coast of North America dating from the last interglacial epoch. Diversification was affected by mass extinctions, as rocky-shore ecosystems expanded and contracted through a combination of species attributed to Archaic, Paleozoic, Mesozoic, and/or Modern biotas. Stromatolites dominated Precambrian rocky shores, but continued as the principal Archaic biota through to the Miocene. The Paleozoic rocky-shore biota is characterized by encrusting inarticulate brachiopods, tabulate corals, and polyplacophorans, as well as ichnofossils representative of boring sipunculid worms (ichnogenus Trypanites)) and acrothoracican barnacles (ichnogenus Zapfella)). Boring bivalves (ichnogenus Gastrochaenolites)), encrusting bivalves (including oysters and rudists), scleractinian corals, and coralline red algae, as well as terebratulid brachiopods, are typical of an enhanced Mesozoic rocky-shore biota. The much expanded biodiversity of the Modern rocky-shore biota is demonstrated by clinging but mobile gastropods, fixed bivalves that adopted byssate and wedging habits, and by balanomorph barnacles. Adaptive innovations played critical roles in the long-term colonization of rocky-shore substrates, but the primary force behind the expansion of rocky-shore ecosystems through geologic time was selective biotic displacement from offshore low-energy to onshore high-energy settings. Rocky coastlines subjected to strong and persistent wave shock are effective “safe places” where species living in the intertidal zone often find refuge from predators and other competitors. This thesis is tested by checking the offshore origins of successful rocky-shore groups including barnacles, bivalves, corals, and coralline red algae. Concepts of keystone species and ecological locking in ancient rocky-shore ecosystems are explored. Latitudinal gradients and other geographic relationships among Pleistocene rocky-shore groups are commensurate with the Recent record, but only vaguely apparent for groups dating from earlier periods such as the Cretaceous. Time intervals for which even the most rudimentary data on rocky-shore biotas are most sparse include the Paleocene, Triassic, and the Devonian.

Sea level and faunal changes during the latest Llandovery and earliest Ludlow (Silurian)

It is now recognized that the late Telychian and early Gorstian sea level changes are, like many others in the Silurian, of world‐wide extent. The 30–50 m deepening events at these times were between 1 and 2 Ma in duration, so melting continental ice caps appear to be the most probable cause. The faunal changes associated with the two events are respectively very close to the base and the top of the Wenlock Series and thus segregate many of the faunas diagnostic of the Llandovery, Wenlock, and Ludlow series. Permanent extinctions (often followed by radiations) are more prevalent in the graptolites, conodonts, and acritarchs, while benthic faunas are more affected by regional shifts in community distributions. This means that, in the benthos, slowly evolving lineages are the only reliable guides to time. Such phyletic evolution, however, appears to have been unaffected by sea‐level events.

Continental island from the Upper Silurian (Ludfordian Stage) of Inner Mongolia: Implications for eustasy and paleogeography

An unconformity between the Silurian Xibiehe Formation and Ordovician igneous rocks marks the perimeter of a small paleoisland near Bater Obo in north-central Inner Mongolia, 180 km northwest of the provincial capital of Hohhot. The stratigraphic position of the lower part of the Xibiehe Formation is correlated by means of conodonts with the upper part of the Ancoradella ploeckensis Zone in the basal Ludfordian Stage (corresponds to mid-Ludlovian Epoch, ca. 421 Ma). Elongate in plan (610 m × 200 m), the exhumed diorite core rises 30 m above the lowest elevations of surrounding Silurian strata. Paleoshores along the principal axis of the inlier delineate contrasting facies. Robust stromatoporoids are in growth position within silty limestones, some directly encrusting the unconformity surface of the sheltered southeast margin. A basal conglomerate of diorite cobbles and boulders characterizes the high-energy northwest margin. The depositional constraints and timing of transgressive facies associated with this continental paleoisland have implications for the eustatic and paleogeographic history of the parent Sino-Korean plate. Burial of the island corresponds to the beginning of a global rise in sea level that peaked in late Ludlovian time. Our interpretation of windward and leeward facies requires an approximate 90° clockwise rotation of the parent plate to accommodate the dominant pattern of low-latitude trade winds and storms.

Tempestites recorded as variable Pentamerus layers in the Lower Silurian of southern Norway

The brachiopod Pentamerus oblongus is especially abundant in the Lower Silurian RytterAker Formation, which occurs widely throughout the Oslo region in southern Norway. Alternating thin limestones and shales typically occur in a shallowing-upward sequence ending in a massive grainstone often rich in tabulate corals and stromatoporoids. In the lower beds, colonization by pentamerid populations was enhanched by prior emplacement of an orthotetacean shell pavement over the clastic sea bed. Individual Pentamerus size decreases with increasing disturbance of populations or their fragmentation as lag deposits nearer the stratigraphic level of the massive grainstone. The frequency of specific storm events and the changing depth to the sea bed may be indexed according to the average size of pentamerid shells in a given population and the degree to which the population is disturbed. Preservation of prominent growth lines (assumed to be annual) suggests storms stirred the sea bed in deeper waters on an 8-10 year cycle. In shallower waters, storms more regularly disturbed the sea bed every 2-3 years. These data corroborate the interpretation that the Pentamerus community inhabited marine waters below fair-weather wave base, but still within reach of storm wave base.

Encrusting corals on a latest Ordovician to earliest Silurian rocky shore, southwest Hudson Bay, Manitoba, Canada

Paleozoic tabulate corals are generally thought to have been free standing, a flattened disc-shaped to dome-shaped morphology providing a degree of stability in shallow-water, high-energy environments. The ability to encrust has previously been suggested by patterns of competitive overgrowth in certain species. Definite proof of encrustation by favositid corals is exhibited in an extraordinary example of an ancient rocky shore exposed for 350 m on Hudson Bay near Churchill, Manitoba. Carbonate strata attributed to the Upper Ordovician Port Nelson or Lower Silurian Severn River Formations locally transgress a massive Precambrian quartzite. An ancient shoreface is clearly marked by large, smoothly eroded boulders of the dark quartzite, commonly 2–10 m in diameter. The boulders are buried in coarse carbonate debris, but corals up to 20 cm in diameter are found cemented directly onto the surface of some boulders. Deep pitting of many boulders to a depth of 2–3 cm was contemporaneous and may have been promoted by unpreserved encrusters such as sponges or anemones.