Finn Surlyk, Co-leader
Neil Opdyke, Co-Leader
Jaques Laskar, Astronomie et Systemes Dynamiques, Bureau des Longitudes, Paris, F-75014, France
Sidney Hemming, Lamont-Doherty Earth Observatory, Palisades, New York, 10964, USA
Susan Herrgesell, Lamont-Doherty Earth Observatory, Palisades, New York, 10964, USA
Vadim A. Kravchinsky (Ministry of Natural Resources of Russia and Siberian branch of Russian Academy of Science, Institut de Physique du Globe de Paris)
We target the high latitudes during Triassic-Jurassic time to investigate the climate of a hothouse earth, as well as to compare with the high-resolution climate and standard magnetostratigraphic record already obtained from the Newark Basin Coring Project.
A lacustrine record is desired from a deep rift basin because lake basins are known to be sensitive to climate change and thus can record Milankovitch variability. An ideal lacustrine deposit will also have several other important characteristics. The primary requirement is good preservation of paleomagnetic signals. Normal and reverse polarity, in conjunction with the cyclostratigraphy, should allow correlation with that record in the Newark Basin, thereby giving good time constraint to sedimentological, biological, chemical, and other variations. Magnetic measurements will also allow precise description of the paleolatitude of the basin, and especially the drift in latitude.
A rich record of flora and fauna within the basin is also desired, in order to describe the change in climate through time, as well as to distinguish facies changes across the basin. As a corollary to the biotic record, if the Triassic-Jurassic boundary is captured, it may be possible to better describe the extinction at that time.
The recognition of organized climatic cycles within lacustrine sequences in the Newark Basin has raised the fantastic possibility for using this type of observation for absolute correlations among Triassic-Jurassic sedimentary successions. It is known the 400ky cycle is extremely stable. Accordingly with relatively few absolute ages, it is possible to get extremely good relative time variations at the time scale of less than 20 thousand years. This 400 ky cycle is readily recognized in the Newark Basin section, and is expected to be easily identifiable in other lacustrine successions as well.
The hope is that a high latitude record should allow recovery of the main periodic terms of the obliquity signal, and that this can be used to further constrain celestial mechanics. The stable 400ky cycle can be used as a reference frame into which the other periods can be determined. In particular, we desire to examine the obliquity signal more closely because of the recently recognized potential for using the details to understand the length of the day and earth-moon distance. The application is based on the principal that the eccentricity of the earths orbit is disturbed within the plane of the orbit by other planets (g, where the number following is the planet counted away from the sun), and the change in the inclination of the Earths orbit due to disturbance outside the plane (s, as with the g,s). The precession of the earth (p), is
The 40 ky component is the most visible part of the signal, and originates from two terms: p+s3 (41 ky), and p+ s4 (39.6 ky). Even if each term is not resolved individually, the modulation of the component should be clearly visible with the frequency s3-s4 (1.2my at present).
The modulation of eccentricity or of the climate precession with frequency g3-g4 (2.4 my at present) should also be possible. In any cases, this modulation is present in the Newark basin record, which can already be used. The observation of a transition from a 2:1 resonance to a 1:1 resonance between the two modulations s3-s4 and g3-g4 will have the following consequences:
In addition to the 40 ky component, there exists a lower amplitude 53 kyr term of the obliquity. This term, of frequency (p+s6) is of small amplitude, but is isolated. Accordingly it ma be possible to determine its precise period during the Triassic-Jurassic, provided the quality of the record is good enough, and the length is sufficient. As s6 is a very stable orbital period, quantification of this ca. 53 ky period may provide a determination of the precession frequency p with a precision (To be determined). Although more calculations and scrutiny of natural records are needed in order to take advantage of this potentially powerful tool, it appears that from an accurate measure of the obliquity components one may obtain in an independent way both the length of the day and the Earth Moon distance at 200 Ma! To identify with sufficient precision the 53 ky terms one needs at least 5 to 10 my per data points which mean at least 20 My in order to get 3 data points necessary for the determination of the value of p and its derivative.
As previously started it is highly desirable to locate prospective drilling sites at high paleolatitudes. Two areas of Pangea would potentially meet these requirements. The most suitable region of the present northern hemisphere which meets these requirements is the Siberian platform which was essentially on the geographic northern hemisphere geographic pole in Triassic Jurassic times.
There is a number of Triassic Jurassic cross-sections in Siberia. They are situated directly on the Siberian platform or in folded areas surrounded of the platform. Early Middle Jurassic is very well studied in Irkutsk coal basin (south of platform). This basin occupies more then 600 km2 from Lat. 52 degrees to 54 degrees , and Long. 100 degrees to 105 degrees (Figure 1). Rocks are represented by continental lake sediments with coal deposits. Summary thickness of Jurassic is up to 2000 meters. Orientation of bedding is sub-horizontal with very little deviation. Polinology, fishs, insects, phillipode, mollusks, rich flora describe detailed geological age. Sedimentation process took place during stable tectonic environment, that is why rhythm and character of sedimentation is transgressive (in Trans-Baikal region - regressive). It is made stratigraphic correlation of south of Irkutsk platform suites, other parts of Siberia and Trans-Baikal ones. In the northeast part of the platform there are continental and shallow sea deposits. South of the platform was studied by paleomagnetic method and got mainly overprinted directions for paleomagnetic pole calculations. At the same time great circles analysis and a few sites illustrate definitive separation of all directions for two groups: normal and reversal polarity group (Figure 2). Potentially these sediments might be used for future magnetostratigraphy, but more work should be done first. Stratigraphy with fossils is well developed. Sediments (continental and marine) of northeast part of the platform are described stratigraphically, but have never been studied paleomagnetically. There are some outcrops, quarries and mines in this region. In terms of easily achievement south Siberian sections are preferable (there are well developed roads).
Triassic - Jurassic sea sediments are well presented in Verkhoyansk folded zone (to the north east of platform), and Early Middle Jurassic in Upper Amur Depression (Far East of Russia). These sea sediments have very good stratigraphy and paleontological description, but due to paleomagnetic studies have only overprinting component (folding process in Cretaceous).
South of Siberia (Trans-Baikal region) has number of long section (a few thousand meters) of Triassic and Jurassic andesites, andesite-basalts, dacites, tuffs with some sandstone (about 5-15% of section) with fossils. Absolute dating was made (mainly K-Ar) about 10-20 years ago and only in a few very rare sites. Preliminary paleomagnetic data illustrate very clear primary component with two polarities. As these sections are mainly volcanic origin (with some sediments) it might be complicated to analyze paleoclimate changes. But to improve constraining of magnetostratigraphyc scale might be very perspective with using of paleomagnetic and absolute dating methods.
Moreover, there are many Triassic and Jurassic outcrops and sections in Far East of Russia (sediments), most north of Siberia platform (Jurassic margin) and on Omolon block (toward north-east from the platform). All this sections have stratigraphical description with fossils. Majority of paleomagnetic data are 15-25 years old and do not fit present reliability code. Some of new data support existing of primary component in the published outcrops. Probably these sediments might be used for paleoclimate studies too. Sections are a few tens meters. These sections usually are very far away from big villages and it is not very easy to reach them.
Southern Hemisphere prospects
The other region of ancient Pangea that was at high paleolatitutes was the southern part of South America, Africa, Antarctica and Australia. Ancient pre-Triassic rift type basins exist in Central Africa in Tanzania, Zambia, Malawi and Zimbabwe. We believe that these basins have some potential for cyclostratigraphy; however, none has been demonstrated. The Karoo basin in South Africa is a foreland basin and the potential for magnetostratigraphy is good but the potential for cyclostratigraphy is unknown. South America also has a similar stratigraphic and structural history and would appear to have similar problems far than search ?? reveal suitable drilling targets.
It should be notes that Australia is clear at high paleolititudes during the Triassic and Jurassic and has rocks which could potentially yield good cyclostratigraphic and magnetostratigraphic information. At present however we feel that best target for immediate drilling in the East Greenland basin for the reasons given below.
Jameson Land basin, central East Greenland
Although in a transitional mid- to high-latitude setting, the Triassic-lowermost Jurassic succession of the Jameson Land basin in eastern Greenland is an excellent target for scientific drilling, where it may be possible to identify the 40 ka obliquity cycle, the 53 ka cycle. It is additionally a good location to study the effect of latitudinal climatic gradients as well as the nature of the mass extinction at the Triassic-Jurassic boundary.
The Jameson Land basin is an extensive continental rift basin. The succession is well exposed due to Cenozoic uplift on the order of 1-2 km. The strata are flat lying, undisturbed except for minor faulting to the north and the succession accessible by foot or helicopter in most of the area. The continental Triassic-Jurassic section (to the Sinemurian) is 1-2 km thick, and represents near continuous deposition over about 50 my, from the Permian-Triassic boundary. Deposition was dominantly lacustrine and cyclical in the interval of interest. The bulk of the Triassic succession comprises cyclic arid lacustrine red beds (Fleming Fjord Fm.), whereas the Rhaetian-Sinemurian consists of alternating black, deep lake shales and lake delta sheet sandstones (Kap Stewart Fm.).
The lithological change from red beds to black and drab beds with basin margin coals reflects a long-term regional change from arid to humid conditions. The paleolatitude was about 35-40 degrees N. Physical stratigraphy and depositional environments are known in detail, and paleomagnetic studies have been carried out for parts of the red bed succession.
The red bed succession is well suited for further detailed paleomagnetic studies, whereas the Rhaetian-Sinemurian black shale-sandstone succession requires unweathered material, only available by drilling. The red bed cyclicity is complex and likewise requires coring.
The Rhaetian-Sinemurian fluvio-lacustrine deposits contain a world-class macroplant flora which shows a dramatic break at the Triassic-Jurassic boundary. Two floras are identified, the Rhaetian Lepidopteris flora and the Hettangian Thaumatopteris flora. They have a species overlap of only 5%. The sharp floral break has been attributed to the presence of a major hiatus at the boundary or to a mass extinction.
Fieldwork is restricted to July and August and weather conditions are usually excellent. Jameson Land can be reached with scheduled weekly or by chartered flights from Iceland. There are to two airstrips situated in the southern and northern part of the peninsula, respectively. Transport within the area is by helicopter, which is normally available in the summer months. Drill sites can easily be selected and thick succession can be covered at many places by offset shallow core drilling, using the step-like landscape to move down through the strata. Extensive, shallow slim core drilling with helicopter portable drill rig was carried out in the eighties by the former Geological Survey of Greenland mainly in potential source rock units. The Danish Polar Centre in Copenhagen assists with logistical planning, helicopter pool charter, permissions etc.