عنوان
پدید آورنده
موضوع
رده
کتابخانه
محل استقرار
TLpq303747133
انگلیسی
Chronostratigraphic variability in fluvial sequences as revealed by paleomagnetic isochrons:
[Thesis]
L. E. McRae
Examples in Miocene strata from a Central Andean intermontane basin (Salla, Bolivia) and the Northwest Himalayan foreland (Lower Siwalik Group, Chinji Formation, Pakistan)
R. C. Reynolds
Dartmouth College
1989
432
Ph.D.
Dartmouth College
1989
The chronologic framework provided by magnetostratigraphic studies in terrestrial Neogene strata can be exploited at various levels of time resolution ranging from 10-10 years, and in complement with stratigraphic and sedimentologic studies, provides a powerful tool that can be used to identify, assess, and quantify both temporal and spatial variability in sediment accumulation. Chronostratigraphic variability is documented and evaluated in two unrelated Miocene stratigraphic sequences that represent different terrestrial environments and tectonic settings: the Salla Beds of Bolivia represent distal floodplain and lacustrine environments of an intermontane basin situated within the Eastern Andean Cordillera, and the Lower Chinji Formation of Northern Pakistan represents cycles of channel and floodplain deposition of a major throughgoing river system in the foreland basin developed along the southern margin of the Himalayas. In both sequences, magnetic polarity stratigraphies have been developed in multiple vertical sections that have been correlated along strike. Paleomagnetic isochrons traced throughout the sequences provide a chronologic framework in which to identify temporal variability (unsteadiness) and spatial variability (non-uniformity) in sediment accumulation. This analysis is accomplished by constructing simulated accumulation histories for time-bounded stratigraphic intervals using estimates of deposition rates for various fluvial facies, considering the effects of erosion, and partitioning an estimated time of hiatus between depositional episodes. Unsteadiness is quantified by comparing short term episodic rates with the long term net accumulation rate and is used to compare variability in accumulation over different time spans and demonstrate non-uniformity between coeval stratigraphic intervals. Transitional paleomagnetic directions recording short term (102-10) excursions just prior and subsequent to field reversal that have been observed in many Cenozoic sequences are observed in both the Salla and Chinji strata. The preservation of polarity details at the 10 year scale implies that sedimentation must have occurred at frequent enough intervals to record short term transitional behavior of the geomagnetic field. Such a calibration has significant implications in assessing the real nature of punctuated and gradual transitions observed in fluvial stratigraphic sequences. (Abstract shortened with permission of author.)
Earth sciences
Geology
L. E. McRae
R. C. Reynolds
مطالعه متن کتاب p
[Thesis]
276903
a
Y
