The Late Triassic is characterized by major geologic and evolutionary events in Earth’s history.
During this time, the super continent Pangaea began to break up while numerous faunal groups, including dinosaurs, crocodylomorphs, pterosaurs and mammaliaformes, first appeared. The period terminates with one of history’s largest biotic crisis in the end-Triassic extinction associated with extensive volcanism of the Central Atlantic Magmatic Province. The chronology of these unique geologic and biotic events are critical to understand this period in Earth’s history.
One of the key sections for understanding the paleoclimatic and biological evolution of the Late Triassic is the lacustrine sediments of the Fleming Fjord Group in central East Greenland. These sediments represent an up to 350-m thick cyclic bedded succession deposited in the Jameson Land basin, which was situated in the pre-Atlantic rift basin at the northern margin of Pangaea. The Late
Triassic sediments contain a diverse vertebrate fauna including dinosaurs, early mammaliaforms, phytosaurs, an aetosaur, a pterosaur, fishes, amphibians and turtles. A detailed and reliable chronostratigraphy of the Fleming Fjord Group is essential to incorporate this important vertebrate fauna into global models on biological evolution in the Late Triassic. Moreover, lacustrine sediments are generally excellent recorders of climate change and have often been used to construct records on past climate conditions. Thus, the lake sediments of the Fleming Fjord Group may reflect past climate conditions that can be used to expand our knowledge of Late Triassic climate models and climate dynamics. These sediments are unique due to exceptional exposure and excellent age registry, which allows for an investigation of the paleoclimate and orbital forcing
of the paleoclimate in a world with extremely high atmospheric CO2 concentration.
The collective aim of this PhD study is to strengthen the stratigraphic, temporal, paleoclimatic, and paleoenvironmental interpretations of the vertebrate-bearing lake sediments of the Late Triassic Fleming Fjord Group with special emphasis on a red-bed succession in the upper part of the unit. The lacustrine succession is investigated with detailed field measurements and multiproxy analysis of rock samples from the Carlsberg Fjord Member, analysis of wave ripple orientations
and wave-rippled beds, as well as sedimentary logs and samples from the overlying Kap Stewart Group. The study builds on the existing knowledge of the Fleming Fjord Group, the Rhaetian-Sinemurian Kap Stewart Group, and the Triassic-Jurassic boundary. The specific aims are to: (i) procduce a robust cyclostratigraphic interpretation of the Ørsted Dal Formation anchored to a
magnetostratigraphic correlation with the Newark Basin in order to construct a detailed numerical age model of the vertebrate-bearing sediments and advance interpretations of the orbitally forced climate variations in the Late Triassic; (ii) produce the first radiometric dates of the Fleming Fjord Group that can be used to further constrain magnetostratigraphy, cyclostratigraphy, and biostratigraphy; (iii) refine the interpretation of paleoclimate conditions and especially the
paleowind conditions in the Flemning Fjord Group by analyzing wave-ripple data as proxy for the prevailing wind direction; and (iiii) discribe new vertebrate-bearing rock layers and rock slabs in order to place paleontological findings into a stratigraphic, depositional and temporal context.
The Fleming Fjord Group is composed of the Edderfugledal, Malmros Klint and Ørsted Dal formations; lake sediments form a major component of all formations. Improved cyclostratigraphic interpretation of the Carlsberg Fjord Member in the Ørsted Dal Formation provides a wellfounded, high precision age model utilizing a 405-kyr metronome anchored with magnetostratigraphy. The studied interval corresponds to a total time span of about 4.3 million years and by extrapolation and comparison with magnetostratigraphy it is estimated that the Fleming Fjord Group represents ~11 million years in the Norian. Radiometric dating of microbial
carbonate sediments from the Fleming Fjord Group confirm a Late Triassic age, which is also suggested from paleontological findings and magnetostratigraphy. The radiometric dates may also provide constraints on a sub aerial exposure between the Fleming Fjord and Kap Stewart Groups.
However, the uncertanty of these radiometric dates are too large to be used for anchoring of the magnetostratigraphy and cyclostratigraphy.
The cyclostratigraphic analysis in this study documents long-term climate variations driven by planetary motions. The orbitally forced sedimentary cycles in the Ørsted Dal Formation show long-term cycles with periods of 850 kyr and 1,700 kyr ascribed to the gravitational interactions between Earth and Mars. This indicates that the these long-term cycles remained in the presentday 2:1 resonance state. However, the cycle periods are shorter than predicted by astronomical solutions and indicate chaotic diffusion of the Solar System. These findings may have implication as reference points in calculations of the past motions of the planets in the Solar System. Moreover, the timing of the 405-kyr metronome suggests that the climate response to long eccentricity at mid
latitudes is ~180° out of phase with low-latitude basins. This could be the result of enhanced equatorward and poleward transport of moisture from the mid latitudes during increased orbital insolation. This provides the first emperical evidence for inverted climate response at mid latitudes and may have implications for precise correlation of orbital cycles imprinted in geologic data.
Measurements of wave ripple orientations in the Fleming Fjord Group are used as a paleowind proxy and indicate dominating paleowinds from the SSE and NNW (154.5º and 334.5º) in the Late Triassic with subordinate paleowinds from W or E (270º or 90º). General circulation model results suggest that the North Atlantic area was affected by prevailing summer winds from the NNW and prevailing winter winds from the SSE. This paleowind regime remained stable throughout the deposition of the Fleming Fjord Group (220-209 Ma) and was most likely greatly affected by high mountains east and west of the depositional basin in Greenland and Norway, which also channeled winds in the entire pre-Atlantic rift basin. These windproxy data suggest a paleowind regime not predicted by the newest general circulation models. However, model sensitivity experiments are needed to better understand the climate models sensitivity to the paleotopography in Greenland and Norway.
Collectively, the results of this study contribute with a refined geologic framework for the Late Triassic (Norian) Fleming Fjord Group in central East Greenland and especially shed new light on the paleoclimatic conditions that prevailed during 11 million years of lake deposition at the northernmost part of Pangea.