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List of publications
Forster, A. (1994): Zur Geologie der Umgebung von Sheep Mountain (Wyoming, USA). Unter besonderer Berücksichtigung der Schichtenfolge Mitteljura-Oberkreide. - 175 p., with enclosures. [Master's-Thesis, Bonn University]
(Geology of Sheep Mountain and vicinity (Wyoming, USA), with emphasis on Middle Jurassic to Upper Cretaceous strata)
Forster, A., Irmen, A. P. & Vondra, C. F. (1996): Structural interpretation of Sheep Mountain Anticline, Bighorn Basin, Wyoming. - Wyoming Geol. Assoc. Guidebook, 47: 239-251.
Irmen, A. P., Forster, A. & Vondra, C. F. (1997): Laramide deformation: a single-phase model at local scale, Bighorn Basin, Wyoming (U.S.A.). - Terra Nostra, 97/1: 13-14.
Forster, A. & Irmen, A. P. (1997): Starthilfe durch das R. & M. TEICHMUELLER-Stipendium. - Nachrichten der Deutschen Geologischen Gesellschaft, 61: 72-73.
(Getting started with the help of the R. & M. TEICHMUELLER-Scholarship)
Forster, A., Dean, W. E., Schwark, L., Thein, J., Trappe, J. & Vondra, C. F. (1998): Geochemical Investigations on the Turonian Carlile Shale, Cretaceous Western Interior Seaway, U.S.A. - Erlanger Geol. Abh. Sonderband, 2: 22-23.
Forster, A., Schwark, L., Thein, J. & Trappe, J. (1999): Epikontinentale Black Shales: Geochemische Untersuchungen des mittelturonischen Carlile Shale anhand von Kern-material des Amoco Bounds Core, Kansas (USA). - Terra Nostra, 99/4: 62-64.
(Epicontinental black shales: geochemical investigations on core-samples of the Amoco Bounds Core, Kansas (USA))
Forster, A., Dean, W. E., Schwark, L., Thein, J., Trappe, J. & Vondra, C, F. (1999): Geochemical investigations on cyclicity, paleoenvironment and organic matter deposition of the Middle Turonian Carlile Shale, Western Interior Basin, U.S.A. - Geol. Soc. Am. Abstracts with Programs, 31: A-225. Geochemical Investigations on the Turonian Carlile Shale Cretaceous Western Interior Seaway, U.S.A Forster, A.1; Dean, W. E.2; Schwark, L.3; Thein, J.1; Trappe, J.1 and Vondra, C. F4. 1 forster@geo.uni-bonn.de This project focuses on the control and development of black shale sedimentation and related organic matter deposition within the central part of the Western Interior Basin (WIB) by means of combined organic and inorganic geochemical analysis of sedimentary facies. During Albian to Santonian time three major black shale bearing sequences are recorded in the central part of the WIB in connection with world-wide second-order sea-level fluctuations interfering regionally with subsidence, sediment input and marine circulation patterns in the basin (transgressive-regressive cycles Kauffman (1977) for the Western Interior Seaway: Kiowa-Skull Creek, Greenhorn and Niobrara Cycles). In order to investigate the paleoenvironmental controls on the formation of one black shale deposit, an East-West-transect through the WIB extending from Iowa to Utah is established by six cores of the middle Turonian Carlile Shale. During the Turonian transgression an epicontinental seaway extended meridionally from northern Canada to the Gulf of Mexico, connecting water masses of the polar ocean with those of the subtropical ocean to the South. Adjacent to the rising highland of the Sevier Orogenic fold- and thrust belt the western part of the WIB could be characterized as a foreland basin but to the east the WIB changes into a stable platform area bordered by the North American craton. The Amoco Rebecca Bounds core from Kansas serves as reference section because it is located in the central part of the Middle Turonian WIB and reveals the most marine and undisturbed conditions of sedimentation along the transect. The Carlile Shale represents the regressive upper part of the Greenhorn Cycle and can be divided into three to four members based on different lithology and biostratigraphy. The deposition of lowermost Fairport Chalky Shale Member consisting of marls to calcareous shales took place under distal offshore conditions within the succession at an estimated water-depth of about 200 to 300 meters for the location of the reference core (Sageman et al., 1997). Referring to Rock Eval pyrolysis, the organic matter can be characterized as immature and predominantly marine derived (type II kerogen). The typical white specks and thin laminae occurring in the Fairport Mbr. are composed of planktonic fecal pellets consisting mainly of coccoliths or coccolith debris. The lamination is disturbed from time to time by scattered shell debris or erosional based calcarenite layers. This can be interpreted as the effect of storm induced bottom currents or distal turbidity flows. Upsection from a depth of 255 meters, the lamination declines while siliciclastic and volcanic input as well as the number of erosional layers increases. This lithofacial transition is accompanied by a sudden improvement of source rock quality most evident expressed by the content of total organic carbon (TOC) from an average value of 3,4 (%) below to 4, 7 (%) above 255 m. The deposition of upper Fairport Mbr. took place at increased oxygen deficient bottom water conditions as evident from geochemical and biological indicators. The top of the Fairport represents a transition to shallower water conditions leading to the deposition of the silty shales of the Blue Hill Shale Member. This member of the Carlile Shale contains more terrestrially derived organic matter (type II to III) with still good source rock quality (average TOC: 2,4 %). While regression continued, the sedimentation of the Codell Sandstone Mbr. occurred in an upper shelf to shoreface environment. Related to oxic conditions it contains only small amounts of organic matter (terrestrially derived, type III). At the top of the Codell Sst. Mbr. a thin layer (about 1 m) of very calcareous sandstone occurs that contains lithoclasts of a calcarenite (organic matter properties similar to Codell Sst.). This could possibly be a relict of the Juana Lopez Member of the Carlile Shale. In its type area in Colorado this member is related to the transgression of the Niobrara Cycle. The long-term environmental development during the Blue Hill- and Codell-timespan seems to be linked to the continuous regression of the Greenhorn Cycle. On the other hand, control by a dropping sealevel alone can not explain the more complicated variations in organic matter accumulation occurring in the Fairport Mbr. or small scale TOC-cycles within both the Fairport and the Blue Hill members. Therefore, additional controlling factors have to be taken into account. For instance, eustatically or tectonically triggered short time sealevel fluctuations are not only causing the rearrangement of facies zonations but they can also change the overall pattern of water circulation within the basin. In case of the WIB this could result in variations of water temperatures by means of enforced or reduced mixing of cold water from the north and warm water derived from the southern region. Another possible scenario could be that an increase of precipitation in the western highland area (e.g. due to a change towards a cooler, more humid climate) could result in a higher amount of continental run-off into the WIB. In consequence this would intensify or cause the stratification of the water column and therefore favor conditions of oxygen depletion. Variations in paleosalinity, paleotemperatures and organic matter composition should give clues towards a better understanding of parameters controlling the sedimentation-process. References Kauffman, E. G. (1977): Geological and biological overview: Western Interior Cretaceous basin. - In: Kauffman, E. G. (ed.): Cretaceous Facies, Faunas, and Paleoenvironments Across the Western Interior Basin. - Mountain Geologist, 14: 75-99. Sageman, B. B.; Rich, J.; Arthur, M. A.; Birchfield, G.E. & Dean, W. E. (1997): Evidence for Milankovitch Periodicities in Cenomanian-Turonian Lithologic and Geochemical Cycles, Western Interior U.S.A. - Journal of Sedimentary Research, 67: 286-302. GEOCHEMICAL INVESTIGATIONS ON CYCLICITY, PALEOENVIRONMENT AND ORGANIC MATTER DEPOSITION OF THE MIDDLE TURONIAN CARLILE SHALE, WESTERN INTERIOR BASIN, U.S.A. FORSTER, Astrid, Geol. Institut, Univ. Bonn, Nussallee 8, 53227 Bonn, Germany, forster@geo.uni-bonn.de; DEAN, Walter E., U.S. Geol. Survey, P.O. Box 25046, Denver, CO 80225; SCHWARK, Lorenz, Geol. Institut, Cologne University, Zuelpicher Str. 49 a, 50674 Koeln; THEIN, Jean; TRAPPE, Joerg, Geol. Institut, Univ. Bonn; VONDRA, Carl F., Dept. of Geol. & Atmospheric Sciences, Iowa State Univ., 253 Science Hall 1, Ames, IA 50011 During Albian to Santonian time, three major black shale bearing sequences were recorded in the central part of the Western Interior Basin (WIB) in connection with third-order global sea-level fluctuations. The development of black shale sedimentation and related organic matter deposition in this epicontinental setting were studied by means of combined organic and inorganic geochemical analysis. In order to focus on the spatial and temporal paleoenvironmental control within a single black shale unit, an east-west transect through the WIB extending from Iowa to Utah was established by seven cores of the Middle Turonian Carlile Shale. The Amoco Rebecca Bounds core from Kansas served as reference section because it reveals the strongest marine and most undisturbed conditions along the transect. Here, the lowermost Fairport Chalky Shale Member consists mainly of laminated marlstones. The organic matter is immature and predominantly of marine origin, but a strong freshwater influence is indicated by biomarkers. The silty shales of the Blue Hill Shale Member represent a transition to shallower water under still oxygen-deficient conditions, but with an increased influx of terrigenous organic matter. Finally, the deposition of the Codell Sandstone Member took place in an oxic near-shore environment. The long-term development of the Carlile Shale is linked to the continuous regression during the upper Greenhorn Cycle, whereas the upper part of the Fairport Member shows a sudden increase in organic matter concentration connected with renewed marine conditions during a fourth-order sea-level rise. For this time interval, intensified oxygen-depletion within bottom waters is evidenced by geochemical and biological indicators. Small-scale cycles in the Fairport and Blue Hill Members that are expressed by variations in lithology and geochemistry may have been climatically controlled or were induced by short-term, eustatically or tectonically triggered sea-level fluctuations. BIOGEOCHEMICAL INVESTIGATIONS ON ORGANIC MATTER DEPOSITION DURING THE CENOMANIAN/TURONIAN BOUNDARY ANOXIC EVENT: LIVELLO BONARELLI, GUBBIO, ITALY Astrid FORSTER *, Jaap S. SINNINGHE DAMSTÉ* and Elisabetta ERBA** Widespread organic-rich sediments of Cretaceous
age have been recognized from different marine settings on a global scale,
but are restricted to certain intervals during the Aptian to
Santonian. The mid-Cretaceous period is characterized by the
coincidence of intensified volcanism and oceanic crust production,
peak sea-level highstand and high paleotemperatures, culminating
in the early Turonian. The concept of global "Oceanic Anoxic Events"
(OAEs) was postulated in order to explain time-slices of enhanced
organic carbon burial like that at the Cenomanian/Turonian (C/T)
boundary (OAE 2). They may be linked to periods of oxygen-crisis
in already oxygen-depleted warm oceans and epeiric seas, enforced
either by a sudden increase in primary organic matter production
related to transgressions or the onset of density stratification
due to slowed rates of oceanic circulation. It is believed that
the increase in the atmospheric CO2-concentration is
the main cause for the Cretaceous "Greenhouse"-climate. Therefore,
the black shale deposition related to the OAEs may provide the
feed-back mechanism of the global carbon cycle that is needed to
compensate the large amount of CO2-outgassing linked
to the volcanic and rifting processes. Evidence for the assumption of a major atmospheric
CO2-drawdown by intensive black shale deposition and
thereby induced cooling is provided by investigations on the globally
observed positive carbon-isotope excursion (e.g., Kuypers et al., 1999),
that is expressed both in carbonate and organic matter at the
C/T-boundary. In order to lead to better understanding of the causes
and consequences of this enhanced organic carbon burial event,
the recently established European C/T-network has chosen the OAE 2
for a multidisciplinary research project including combined investigations
on biostratigraphy, paleoecology, petrology, as well as inorganic
and organic geochemical analysis of the sediments. This study is
mainly based on C/T-sections in SE-England, Morocco and the type
section for the C/T-OAE 2 located in Gubbio, Italy. Here, the C/T-boundary
event is represented by the Livello Bonarelli, a regionally
traceable black shale band within a succession of predominately
white limestones below and above. This only one meter-thick condensed
interval consists of organic-rich, carbonate-free marine black shales
intercalated with radiolarian derived, up to sand-sized, siliceous material.
A core drilled at the Gubbio site for C/T-net was sampled in high
resolution, obtaining 50 samples for further analysis. Our contribution
to the joint project will focus on the biogeochemical characterization
of the black shale deposits by biomarker analysis, in order to evaluate
the paleoenvironmental conditions and the organic matter sources.
Based on this, compound-specific d13
C-analysis will be carried out on selected biomarkers by isotope-ratio-monitoring
GC-MS. For those C/T-sections studied, that are also containing a sufficient
amount of carbonate, these results will be combined with carbonate stable-isotope data
to reconstruct the carbon-fractionation factor (e p) for marine
phytoplankton, enabling the recalculation of atmospheric pCO2 and the
observation of its temporal evolution. First investigations indicate that the black shales from
the Gubbio-section are very immature. This is evident from the abundance of
both, 5b- and 5a-
aa-R C27 to C29 regular steranes,
C29 to C32 bb-hopanoids
and a variety of unsaturated compounds like diasterenes (C27 to C29, R-isomers
dominating) and neohopenes. A predominantly marine, phytoplanktonic origin of
organic matter is suggested by the n-alkane distribution, with n-C15 to n-C19 being
most abundant. A minor, but well recognizable input of terrestrial derived organic
carbon is recorded by a strong odd-over-even predominance between n-C23
and n-C35. The relative abundance of 2b-methylhopanoids indicates that cyanobacteria
are an important component of the phytoplankton. The abundance of sulfur-containing
compounds like thiolanes and tetracyclic regular isoprenoid sulfides gives
clues towards higher concentrations of sulfides in the sediment
that are necessary to incorporate sulfur into organic matter.
The wide spectrum of primary biomass contributing to the organic matter
preserved hopefully enables us to evaluate possible changes in the phytoplanktonic
community or the terrestrial ecosystems (e.g., shifts in vegetation from C3 to
C4-type; Kuypers et al., 1999). From the analysis of free and especially
from bound biomarkers, it is expected to gain further clues towards the
development and extension of water-column anoxia. REFERENCES Kuypers, M. M. M., Pancost, R., Sinninghe Damsté, J. S., 1999.
A large and abrupt fall in atmospheric CO2 concentrations during Cretaceous times.
Nature 399, 342-345.
(please click highlighted items to see abstract)
Forster, A., Sinninghe Damsté, J. S. & Erba, E. (2001):
Biogeochemical Investigations on Organic Matter Deposition During the
Cenomanian/Turonian Boundary Anoxic Event: Livello Bonarelli, Gubbio, Italy. -
20th International Meeting on Organic Geochemistry (IMOG), 10. - 14. September 2001, Nancy, France, Abstracts Volume 1: 273-274.
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** University of Milan, Department of Earth Sciences, via Mangiagalli 34, I-20133 Milano, Italy.
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