https://doi.org/10.1130/G45507.1 Manuscript received 20 August 2018 Revised manuscript received 25 January 2019 Manuscript accepted 6 March 2019 © 2019 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 22 March 2019 The largest delta plain in Earth’s history Tore Grane Klausen1,2, Björn Nyberg1, and William Helland-Hansen1 1Department of Earth Science, University of Bergen, Allégaten, 41, 5020 Bergen, Norway 2Petrolia NOCO AS, Espehaugen 32, 5836 Bergen, Norway ABSTRACT marine-influenced parts of river systems (Blum Delta plains host heavily populated and extensive agricultural areas with strong anthropo- and Roberts, 2009; Bhattacharya et al., 2016). genic overprints on the natural evolution of these important landforms. Furthermore, mod- Siliciclastic sedimentation in the TBO started ern delta plains have formed over a short geological time frame, representing immature end with very high sedimentation rates shortly after members to ancient counterparts in Earth’s history—it could thus be argued that these are the Permian-Triassic event (Eide et al., 2017), poor analogues for deciphering the sedimentary rock record. Our present study offers unique resulting in kilometer-thick siltstone-dominated insight into the controls and potential extent of ancient deltas by investigation of the Triassic successions prograding >1000 km into the basin Boreal Ocean, where a large delta plain has been traced across >1.65 × 106 km2. We show by during the Induan (Early Triassic) and creat- comparison that the Triassic Boreal Ocean delta plain is larger than all modern and known ing a relatively shallow epicontinental basin. ancient counterparts. Supply-driven progradation of this delta system proceeded uninter- After a reduction in sediment influx during the rupted on a 106 yr scale, indicating relative sea-level stability during this period—in support Olenekian and part of the Anisian, indicated by of a Triassic Greenhouse without pronounced glaciations. Reconstructed paleo-bathymetric the slight backstepping of sedimentary pack- relief shows the Triassic Boreal Ocean to have been one order of magnitude smaller than ages relative to the Induan (Fig. 1C), sediment modern equivalents, explaining its vast extent. Despite its extent, the delta plain shows similar supply resurged in the Middle Triassic Ladinian geomorphological characteristics to many modern delta plains, supporting their validity as interval. Because the northwestern boundary of analogues to the ancient, although scales might vary significantly. this system cannot been defined, it is impossible to say how far the system prograded, but it had INTRODUCTION resuming the depositional style characterizing to prograde >500 km to cover the entire basin Understanding the character and develop- foregone periods with prolonged highstands, with deltaic sediment. This westward migration ment of delta plains is crucial to constraining such as the Triassic, when low-gradient delta of its main depocenter could be explained by past eustatic sea level (Haq et al., 1987; Miller plains developed over the large marine areas the Ladinian humid interval (Bernardi et al., et al., 2005), paleogeographic reconstructions that are today occupied by continental shelves. 2018) and later Carnian pluvial events (Hochuli (Miller et al., 2013), past climate (Hochuli To investigate the character and extent of and Vigran, 2010) as possible climatic drivers and Vigran, 2010), and the evolution of life a large-scale delta plain unaffected by human that facilitated resurgence in sediment supply (Woodroffe et al., 2006; Greb et al., 2006). interaction, we use seismic reflection data and to the basin. Our understanding of the nature and character well logs to study the subsurface succession Semi-parallel subsurface seismic reflec- of ancient deltaic depositional environments is of the Triassic Boreal Ocean (TBO; Fig. 1). tions are tied to specific periods within the Tri- complicated by how modern delta plains have This succession is characterized by a large assic according to biostratigraphic information developed in an anomalous Holocene highstand (hundreds-of-kilometers areal extent) deltaic (Vigran et al., 2014) and are traceable throughout period with strong anthropogenic influence. For river system of Carnian age (237–227 Ma) the greater Barents Sea basin. Within the Mid- example, sediments trapped upstream by dams across the entire present-day Barents Sea and dle to Upper Triassic (Ladinian to early Norian) combined with bank-stability measures and a is also exposed in outcrops in islands along the Snadd Formation, each progradational package rising global sea level (Syvitski et al., 2009) uplifted northern flank of the basin (Klausen approximates 2–5 m.y. (Paterson and Mangerud, result in net land loss downstream (Blum and et al., 2015). We consider the overall extent of 2017), and within these discrete rock intervals, Roberts, 2009)—directly affecting the shape the TBO delta plain in relation to ancient and characteristic contrasts in acoustic properties of delta plains (Syvitski and Saito, 2007). With modern analogues, and discuss the exceptional enable identification of geomorphological fea- global sea level expected to rise (Rahmstorf, circumstances required to produce the largest tures down to ~15 m thickness in three-dimen- 2007) and delta plains continuing to subside delta plain in Earth’s history. sional (3-D) seismic data, and are used together (Syvitski et al., 2009), present geomorphologi- with core and well logs to interpret fluvial and cal characteristics of deltas are being affected CHARACTERISTICS OF THE TRIASSIC interbedded shallow marine depositional envi- by extreme and anomalous effects of anthropo- BOREAL OCEAN DELTA PLAIN ronments. Three-dimensional seismic data show genic interference. Given that this interference A delta plain is defined as the coastal areas large-scale channel belts up to 25 km wide and is not counteracted, the present interglacial high- with a common gradient profile, controlled by >50 m thick with pronounced lateral accretion stand could be sustained (Archer and Ganopol- backwater-length, toward a proximal knickpoint surfaces in the eastern part of the basin, formed ski, 2005) while deltas will be prevented from between the alluvial plain and the fluviatile to during the early Carnian interval (Fig. 2). This is CITATION: Klausen, T.G., Nyberg, B., and Helland-Hansen,W., 2019, The largest delta plain in Earth’s history: Geology, v. 47, p. 470–474, https:// doi.org /10 .1130 /G45507.1 470 www.gsapubs.org | Volume 47 | Number 5 | GEOLOGY | Geological Society of America Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/5/470/4680578/470.pdf by guest on 24 September 2021 Figure 1. Area and strati- A B Stratigraphy Lithostrat. Fm. pronounced eustatic sea-level lowering (Reijen- graphic context of study, Greenland North Pole o Ma WE o 0 E Rhaetian stein et al., 2011). Large parts of LGM delta Barents Sea region. 80 N 40oE A,B: Regional setting of 208 Norian plains could therefore be net degradational. To uholmen Franz Josef Upper modern Barents Sea (A), Svalbard Fr avoid underestimation, the full possible extents oo Land 227 hosting Boral Ocean in 7080 NN Carnian of these LGM delta plains are considered (see 237 Triassic, a period when FiGreenlang. 2 d Barents Sea Snadd the Data Repository). We exclude the arctic iassic 7 Ladinian basin was dominated 0 0° Novaya Tr ° 242 N 20° Barents and Kara shelves from LGM estimates by several phases of Svalbard C Zemlya Middle Anisian 40 Kobbe ° Norway 247 because they were characterized by grounded delta progradation (B). r Olenekian rt C: Regional cross sec- Barents ve ice sheets during glaciations (Jakobsson et al., Klappmyss Russia 251 400 km Se Lowe Induan Ha 2 Urals tion shows continuity of 0 a ° 2016). The largest polar LGM shelf is located study interval (pink box Study interval Devonian-Permian Oshore marine Non-marine in the Chukchi Sea, covering ~8.26 × 105 km2 or within Carnian interval Basement Late Cretaceous Intrusions Shallow marine in B)—stretching beyond C 7324/7-1S Flattened on Top Jurassic approximately half the size of the conservative limits of our data set to TBO delta plain estimate (Fig. 4B). Outside of west and toward Novaya polar regions, the LGM shelf of the Gulf of Car- Zemlya in east. Well pentaria (Australia) represents an area of 9.01 × 7324/7-1S was one of sev- 5 2 eral wells used to tie the 10 km , and the Yellow Sea represents an area of stratigraphic intervals to 8.56 × 105 km2. The Sunda shelf is also large but regional seismic profiles 200 ms 500 km comprised two distinct paleo-deltaic draining and 3-D seismic cubes systems from the north and south (Reijenstein (data were made available et al., 2011; Sathiamurthy and Voris, 2006), rep- by the Norwegian Petroleum Directorate). The regional seismic section is flattened on the 5 2 5 regional seismic reflector tied to the top of the Jurassic package. Black lines are formation resenting areas of 8.81 × 10 km and 4.98 × 10 boundaries. Triassic strata are marked with purple fill; Jurassic with blue fill. Westward migra- km2 respectively, amounting to a combined area tion of the main depocenter is initiated in Middle Triassic Ladinian interval, representing the of 1.38 × 106 km2. Despite our overestimates of lower part of the Snadd Formation. Lithostrat.—lithostratigraphy; Fm.—formation. the extents of LGM deltas, the TBO delta plain out-scales all. characteristic of meandering river systems form- deposits attests to steady generation of accom- Estimating the size of net-aggradational ing in proximal parts of delta plains. The late modation (Fig. DR2 in the Data Repository) delta plains in the rock record is challenging Carnian shows similar but narrower channelized without periods of substantial degradation. because discrete delta plain boundaries are not deposits, with both delta plains stretching across This is an important characteristic of the TBO: readily constrained and commonly, due to post- the basin (Klausen et al., 2015).