Ancient Epicontinental Seas Research Articles

Boron proxies record paleosalinity variation in the North American Midcontinent Sea in response to Carboniferous glacio-eustasy

Abstract Salinity variation in ancient epicontinental seas has long been challenging to reconstruct. We determined salinity variation in the Late Pennsylvanian North American Midcontinent Sea based on normalized boron concentrations (B/Ga) and isotopic compositions (δ11B) of bulk shale. We analyzed the Stark Shale Member of the Missourian Stage Dennis Limestone cyclothem in the Charleston core (CC) from the Illinois Basin (eastward, more proximal) and the Iowa Riverton core (IRC) from the Midcontinent Shelf (westward, more distal). Both cores exhibited the same pattern of secular variation, with lower B/Ga and δ11B values at their base and top, recording less saline conditions during lower sea-level stands, and higher values in their middle part, recording more saline conditions at highstands—a pattern that conforms to conventional sequence stratigraphic and glacio-eustatic interpretations of Midcontinent cyclothems. At equivalent stratigraphic levels, B/Ga and δ11B values are systematically lower at CC relative to IRC, reflecting the generally lower salinity conditions of the Illinois Basin relative to the Midcontinent shelf, which is consistent with greater freshwater influence from the east due to heavy river runoff into the Appalachian foreland basin. Our findings serve to demonstrate the utility of paired B concentration and isotope data for paleosalinity analysis of deep-time shale formations.

Comments on “Paleosalinity determination in ancient epicontinental seas: A case study of the T-OAE in the Cleveland Basin (UK)” by Remirez, M. N. and Algeo, T. J.

Comments on “Paleosalinity determination in ancient epicontinental seas: A case study of the T-OAE in the Cleveland Basin (UK)” by Remirez, M. N. and Algeo, T. J.

Numerical modelling of tsunami propagation with implications for sedimentation in ancient epicontinental seas: The Lower Jurassic Laurasian Seaway

Tsunamis are frequent events in modern marine environments but evidence of their passing in subtidal ancient epicontinental sea deposits is elusive. It has been suggested that this is due to mis-identification or poor preservation potential. Herein a numerical modelling approach is used to show that tsunami propagation in one large ancient epicontinental sea was hindered by the damping effect of shallow bathymetries and reflection, refraction and diffraction from emergent landmasses. The Imperial College Ocean Model (ICOM) is used for this study and is first validated against data from the Sumatra–Andaman Tsunami of December 2004. A palaeobathymetric dataset is then presented for the Hettangian (Lower Jurassic) Laurasian Seaway with idealised tsunami sources situated on the continental shelf and within the adjacent oceanic basin. Results show that tsunamis forced from within ocean basins adjacent to the epicontinental sea are rapidly attenuated over the continental slope and fail to propagate great distances onto the shelf. Similarly, the sedimentological effect of tsunamis forced from within the epicontinental sea is also restricted. It is concluded that tsunami deposits in ancient epicontinental seas are most likely to occur in relative proximity to the source region and this must contribute to their scarcity in the geological record.

CIRCULATION IN LARGE ANCIENT EPICONTINENTAL SEAS: WHAT WAS DIFFERENT AND WHY?

At times in the geological past, vast epicontinental seas flooded the continents. Almost everything we know about pre-Jurassic (200 million years ago) marine life and environmental change originates from such settings. Most of the truly oceanic sediments deposited before this time have either been subsequently subducted or metamorphosed (Allison and Briggs, 1993). An understanding of ancient epicontinental seas is clearly essential to our interpretation of past ecological and environmental change. Epicontinental seas, however, really have no suitably scaled modern counterparts. They were typically shallow, on the order of 10 to 200 m deep, but of vast extent, covering areas of up to ∼106 km2 (Wells et al., 2005a, 2005b). Such modern seas as the Baltic are of similar depths but are considerably smaller. Without suitably scaled modern analogues our understanding of these important water bodies is handicapped. Since the depth and scale of these ancient seaways makes them so different, their response to storm and tidal waves has been questioned (Hallam, 1975). Specifically, it has been argued that wave energy was attenuated in ancient epicontinental seas by the large distances traveled by waves (Keulegan and Krumbein, 1949; Shaw, 1964; Irwin, 1965). This contention can be rejected in the case of wind and storm waves on the basis of wave height data from modern isolated seas. Wind waves are affected by fetch, duration, and intensity of wind (e.g., see Jonsson et al., 2002, 2005). Substantial waves are regularly documented in even the small epicontinental seas of today. Wave heights—crest-to-trough height—of larger than 6 m are documented in the Baltic Sea almost annually. They result from strong winds (15–20 m/s) blowing over deep water for at least 6 hours. Annual storms can generate wavelengths of 80 m, leading to sediment reworking down to 40 m water …

Local Controls on Carbon Cycling in the Ordovician Midcontinent Region of North America, with Implications for Carbon Isotope Secular Curves

Abstract The carbon isotope record from ancient epicontinental seas may contain much more of a local-scale carbon cycling signal than is generally appreciated. A unique opportunity exists to examine this issue in the case of the Late Ordovician Mohawkian Sea of eastern Laurentia, where the Millbrig K-bentonite stratigraphic framework has been used to delineate a time slice at 454 Ma, extending over ~ 1,500,000 km2 of the eastern United States. Across the time slice, carbonate and organic carbon δ13C vary by 4.5‰ and 7.5‰ respectively, a spatial variation that is as large as temporal (secular) changes in epeiric-sea δ13C that have been reported in the past. These new data are considered in the context of geographic variations in lithological, biological, and other geochemical sediment characteristics. Collectively, these sediment properties distinguish regions of the Mohawkian Sea which likely differed in terms of the nature and relative importance of carbon cycling processes. Water-column depth and structure, and barriers to free exchange of water across the Mohawkian Sea, may have been overarching factors in the development of these regions, raising the possibility that changes in circulation patterns, such as those caused by sea-level change, played a role in driving secular carbon isotope excursions by changing the rate of exchange of dissolved inorganic carbon between water masses. If the observed effects of local carbon cycling on the distribution of Mohawkian Sea δ13C were commonplace in ancient epicontinental marine environments, it would imply that local-scale carbon cycling may have left a nontrivial imprint on epeiric-sea records of secular variations in δ13C, in addition to the imprint left by changes in the global carbon cycle. This may have contributed to the broad scatter in δ13C values observed in the Paleozoic portion of the global carbon isotope secular curve.

Large sea, small tides: the Late Carboniferous seaway of NW Europe

Debate surrounds the extent of tidal influence in Palaeozoic shallow epi-continental seas. In the absence of analogical reasoning, numerical modelling provides a quantitative means of investigating tidality in the geological record. Herein a new finite element model, tested for accuracy on the present-day Mediterranean, is used to predict an exceedingly low tidal range ( <10 cm) in the epi-continental seaway that covered much of NW Europe during the Late Carboniferous. This small bulge may have been amplified to c . 1 m in estuaries, leading to the localized deposition of cyclic rhythmites, agreeing with geological observations. Extremely low tidal ranges in ancient epi-continental seas may be one mechanism to prevent water-body mixing, enhancing stratification and promoting anoxia.

Tidal-Bundle Sequences in the Jordan Sandstone (Upper Cambrian), Southeastern Minnesota, U.S.A.: Evidence for Tides Along Inboard Shorelines of the Sauk Epicontinental Sea

ABSTRACT This study documents for the first time tidal bundling in a lower Paleozoic sheet sandstone from the cratonic interior of North America, providing insights into the hydrodynamics of ancient epicontinental seas. The Jordan Sandstone (Upper Cambrian) in the Upper Mississippi Valley contains large-scale planar tabular cross-sets with tidal-bundle sequences, which were analyzed in detail at an exceptional exposure. Tidal-bundle sequences (neap-spring-neap cycles) were delineated by foreset thickening-thinning patterns and composite shale drapes, the latter of which represent accumulations of mud during the neap tides of neap-spring-neap tidal cycles. Fourier analysis of the bundle thickness data from the 26 measurable bundle sequences revealed cycles ranging from 15 to 34 bundles per sequence, which suggests a semidiurnal or mixed tidal system along this part of the Late Cambrian shoreline. We extend the tidal interpretation to widespread occurrences of the same facies in outcrops of lesser quality, where the facies is recognizable but too few bundles are exposed for tidal cycles to be measured. By doing so, this study shows that tidally generated deposits have a significant geographic and temporal extent in Upper Cambrian strata of central mid-continent North America. The deposition and preservation of tidal facies was related to the intermittent development of shoreline embayments during transgressions. The tidally dominated deposits filled ravined topographies that were repeatedly developed on the updip parts of the shoreface. Resulting coastal geomorphologies, accompanied perhaps by larger-scale changes in basinal conditions and/or configuration, led to changes in depositional conditions from wave-dominated to tide-dominated. Outcrops of the Jordan Sandstone tidal facies in the Upper Mississippi Valley represent the farthest inboard recorded transmission of ocean-generated tides in the Laurentian epicontinental seas, demonstrating that tidal currents were significant agents in the transport of sand along the far cratonic interior shorelines of Cambrian North America. The results of this study improve the facies-level understanding of the genesis of sheet sandstones. Furthermore, tidalites documented here occur in a specific position within a sequence stratigraphic architecture for the Jordan Sandstone. This provides a framework to compare these ancient deposits and processes to younger (e.g., Carboniferous) epicontinental systems where stratal and sediment dynamics are better documented.

Ammonite morphotypes as indicators of oxygenation in a Cretaceous epicontinental sea

An examination of ammonite morphotypes through stratigraphic sections of the Greenhorn Cyclothem of the Cretaceous Western Interior suggests that they may be useful indicators of oxygenation within ancient epicontinental seas. Because some morphotypes appear to reflect a nektobenthic and others a pelagic habitat, the presence of specific morphotypes may be used to indicate benthic and water-column oxygenation, respectively. At the same time, their absence may provide a valuable indicator of dysaerobic to anaerobic conditions that may have promoted the development of petroleum source rocks. □Ammonites, anoxia, Cretaceous, Greenhorn Cyclothem, western Interior.

North American Paleozoic oceanography: Overview of progress toward a modern synthesis

Three fundamental questions have confronted paleoceanographers from the beginning of their North American explorations. What was the size and timing of ancient epicontinental seas: large and long‐lasting or small and brief? What characterized the distribution of biotas and sediments at any one point in time: a multitude of complex facies patterns or a more spacially homogeneous cover? What promoted continental foundering: eustatic changes in sea level or relative changes in sea level brought about by regional tectonics? These questions have been debated by North Americans since the middle 1800s in response to various new insights, usually coming from abroad but often elaborated into substantial contributions of equal standing. Contemporary facies zones in Mediterranean biota found by the Englishman E. Forbes attracted the notice of geologists as early as 1844. C. Whittlesey was among the first to apply the bathymetric scheme of Forbes to the interpretation of American Paleozoic strata in 1851. The outstanding “native” innovation of the period was J. Hall's geosyncline concept, which is reflected in the earliest map of Paleozoic North America made by T. C. Chamberlin in 1881. Another wave of influence spread from the late 19th century work on stratigraphic facies patterns by the German J. Walther. A. W. Grabau is best remembered as Walther's foremost American champion against the formidable layer‐caker E. O. Ulrich in the first decades of the 20th century, but he also made pioneering contributions of his own on Paleozoic sea level studies and global paleogeographic reconstructions. Charles Schuchert was the consummate paleogeographer of this period. Meanwhile, the term “cyclothem” was coined by J. Marvin Weller in 1930 for recurrent Carboniferous strata in Illinois. Controversy fast erupted over a glacial as opposed to tectonic mode of origin for these cycles. In 1964, A. B. Shaw restimulated interest in Paleozoic oceanography through his reformulation of Walther's ideas. Novel additions included a model for epeiric sea carbonates and the methodology for graphic correlation. Invoking studies by C. G. J. Petersen on recent communities in Danish waters, A. M. Ziegler added new emphasis to paleobathymetry based on fossil communities. As was in Grabau's case, this interest eventually led to larger concerns with global paleogeography. Although the same questions are still debated today, the foregoing periods set the stage for a modern, more interdisciplinary approach to Paleozoic oceanography.