Basin Evolution

Roy W. Schlische, Co-Leader, Rutgers University, Piscataway, NJ, USA Martha O. Withjack, Co-Leader, Mobil Technology Company, Dallas, TX, USA
James A. Austin, University of Texas-Austin, Austin, TX, USA
David E. Brown, Canada--Nova Scotia Offshore Petroleum Board, Halifax, N.S., Canada
Juan Contreras, CICESE, Ensenada, B.C., Mexico
Elizabeth Gierlowski-Kordesch, Ohio University, Athens, OH, USA
Lubamir F. Jansa, Dalhousie University, Halifax, N.S., Canada
Maryanne L. Malinconico, L-DEO, Columbia University, Palisades, NY, USA
Joseph P. Smoot, U.S. Geological Survey, Reston, VA, USA
Robert P. Wintsch, Indiana University, Bloomington, IN, USA


Science Issues

The most important science question to be addressed by a coring transect of Pangea is: What are the processes and products of the amalgamation and breakup of a supercontinent? Within the realm of tectonics and basin evolution, our breakout group identified two important issues: the first concerns the evolution of rift basins and rift zones; the second deals with the rift-drift transition.


Rift Basin/Rift Zone Evolution:

Fault-bounded rift basins and the structures they contain are the upper-crustal manifestation of continental extension. As such, these basins and the rift zones containing them allow us to address the following important questions: (1) What can the rifting process tell us about lithospheric and crustal rheology, strain rates, and heat flow during extension? (2) How do the dimensions of rift basins and the rift zone change through time, and what does this tell us about the nature of fault growth, fault linkage, and the localization of strain? (3) Does the presence of multiple unconformity-bounded tectonostratigraphic packages imply that extension is episodic, or are these packages the result of the relatively localized processes of fault growth and linkage? (4) Is extension represented by older tectonostratigraphic packages related to orogenic collapse or supercontinent breakup?

The sedimentary fill of rift basins represents the complex interaction among basin capacity (accommodation space), sediment supply, available supply of water, and, in some cases, eustatic sea-level change. The continuous record supplied by coring, coupled with outcrop and seismic studies and basin modeling, allow us to address the following important questions: (1) What are the relative controls of tectonics and climate on stratigraphy? (2) How do geologic structures (both rift structures and older, preexisting structures), drainage patterns, and provenance control the distribution of facies in the basin fill? (3) What process or processes are responsible for the tripartite stratigraphic packages common to so many non-marine rift basins?

The Triassic-Jurassic Pangean rift system covers an enormous geographic area, allowing us to address these questions concerning the spatial variation in structural and depositional aspects: (1) Did rifting initiate at the same time along the margin? (2) Were there variations in the duration of rifting along the margin? (3) Are tectonostratigraphic wedges related to a temporal progression of the rifting process or to a geographic superposition of different sets of extensional basins? (4) How much of the extensional record is actually recorded in the basin fill? The latter can be addressed through fission-track analyses and thermal-maturation studies.


Rift-Drift Transition

The rift-drift transition marks the phase where intraplate extension gives way to interplate separation and seafloor spreading. The first set of important science questions concern the relationship of CAMP igneous activity to rifting, drifting, and seaward-dipping reflectors (SDR's): (1) Is CAMP related to initial opening of Atlantic? What is the age of CAMP, post-rift unconformity, and SDR's relative to breakup (initiation of seafloor spreading)?

Many Pangean rift basins have undergone inversion, resulting in contractional reactivation of extensional faults, the formation of new thrust faults and folds, and uplift and erosion. Questions related to this topic are: (1) How does rift geometry affect inversion structures? (2) When did inversion occur? (3) Why did inversion occur?

The final set of questions concern spatial variations along the length of the rifted margin and from the landward to the seaward edge of the rifted margin: (1) Was the rift-drift transition synchronous or diachronous along the central Atlantic margin? (2) Does inversion affect the entire width of the margin, and, if not, why should this be so?


Requirements for Basin Evolution and Tectonic Studies

The successful Newark Basin Coring Project (NBCP) has demonstrated that it is possible to extract information on basin evolution and tectonics from core data. Based on the results of NBCP, any potential coring target must satisfy the following requirements: (1) long, continuous record; (2) availability of information from both the depocenter and basin margins; (3) outcrop-control; (4) seismic coverage or potential for additional seismic coverage; and (5) geographic accessibility. Based on the science questions listed above, a coring target should also (1) contain known tectonostratigraphic packages; (2) contain known inversion structures; (3) contain known CAMP basalts; and (4) expand the geographic coverage beyond that of the NBCP.


Coring Targets

Based on the science questions and coring requirements outlined above, we have identified two attractive coring targets.


Fundy Basin

The Fundy basin, located in New Brunswick and Nova Scotia, Canada, is a very large rift basin with excellent exposure in coastal outcrops located on the margins of the basin. This will allow us to determine variations in stratal thickness and facies from the basin margins to the cored depocenter. There is excellent outcrop evidence and limited seismic evidence for multiple tectonostratigraphic packages. Paleomagnetic evidence suggests that Permian strata are present in tectonostratigraphic package I. These packages will allows us to investigate the early rifting history and provide constraints on evidence for pulsed extension. We can also further constrain when contractional deformation ended and when extensional deformation began. The Fundy basin also contains known post-CAMP inversion structures. How are these structures related to synrift and prerift architecture, which is controlled in part for Carboniferous-Permian transtension followed by transpression? In addition to the Triassic-Jurassic boundary, the basin also contains a very thick post-CAMP basalt Early Jurassic-age stratigraphic section, which is poorly known from outcrop studies and which will constrain when rifting ended in this basin.

Because most of the basin is located underneath the Bay of Fundy and the Minas Basin, lots of seismic data of variable quality are available. Some of these data can be re-processed, and it is relatively straightforward to acquire additional seismic data, including 3-D seismic data, which will provide 3-D data on basin geometry and its stratigraphic architecture. The proposed new core data can be tied into various industry drill holes and cores, which will provide additional constraints on basin geometry and depositional architecture, which are necessary for basin-modeling studies. Because of intense industry activity on the continental margin, the Fundy basin coring project offers the potential for industry partnership and ready access to the infrastructure needed for coring and acquisition of new seismic data.

The conjugate margin for the Fundy basin is located in present-day Morocco. The Argana basin contains strata very similar to those in the Fundy basin. Tectonostratigraphic packages are also present in the Argana basin. Are these packages the same age on both margins? Are the unconformities between the packages of broadly similar ages? The Moroccan basins also contain a substantial postrift section which potentially allows us to better constrain the age of the inversion structures along this part of the margin. Both the Fundy and Moroccan basins exhibit more arid facies than those found in the Newark basin. This will allow us to explore latitudinal variation along the rifted margin. In this more arid setting, sediment influx is not primarily by moving water; there are important eolian and chemical inputs.


Southeastern U.S.

The southeastern United States is the only place along the rifted margin where previous studies have postulated a direct connection between offshore SDR's and the onshore but subsurface Clubhouse Crossroads basalt via the J-reflector. This is also potentially the only place on the margin where the SDR's may be the same age as CAMP. If so, the volume of igneous material related to CAMP is enormous. Additional subsurface basalt flows extend as far west as Texas. Seismic data show that inversion structures affect the subsurface basalt flows. The transect has two main objectives: (1) Drill through the Clubhouse Crossroads basalt, the underlying postrift strata, and the underlying synrift section. This will help put the Clubhouse Crossroads basalt in a proper stratigraphic architecture. When did rifting begin and when did it terminate? When did inversion begin? (2) Core the SDR's in the offshore (as well as the overlying postrift section). By dating the Clubhouse Crossroads basalt and the SDR's, we can constrain the absolute and relative timing relationships among igneous activity, the rift-drift transition, and the initiation of inversion.

The southeastern U.S. is also of interest because rifting appears to have ended earlier and inversion began earlier here than in the northeastern U.S. and southeastern Canada. Whereas the northeastern U.S. and southeastern Canada contain NE-striking dikes of probable Early Jurassic age, the southeastern U.S. contains N- and NW-striking dikes of probable Early Jurassic age. Are these N- and NW-striking dikes related to a change in the stress regime from rifting to drifting? Or are they related to a complex stress field resulting from the separation of Africa from North America and South America from North America?