Validation of Sediment Accumulation Regions in Kansas, USA
357
tinct genetic stratigraphic units resolves km-sized regions containing similar
sediment accumulation (sediment accumulation regions, SARs). Rhombic and
arcuate shapes of these SARs correspond closely to gravity and magnetic trends
and lineaments and known structure in the Precambrian basement.
A gamma-ray cross-section reveals that SAR boundaries affect all scales of
stratigraphic units including GS, GSU, and smaller-scale stratigraphic units and
lithofacies. In particular, SAR boundaries control the location of prominent carbonate margins consisting of single and stacked GSUs. Recurrence of carbonate
margins at the same location in different GSs suggests consistency in the site of
sediment starvation and drowning of the shelf. Moreover, the linear trends of
these boundaries and their correspondence with basement structure strongly
links them to sites of basement reactivation. Sediment accumulation regions appear to be validated from this investigation.
GSs are defined as coherent packages of related GSUs. They exhibit abrupt
transgression and more gradual regression, characterized by backstepping of
carbonate-dominated strata and lateral accretion of the same. The duration of
a GS is estimated to be approximately 1.5 Ma (Watney et al. 1995) and is of duration similar to episodic foreland basin subsidence resulting from thrusting
and downloading along collisional margins (Quinlan and Beaumont 1984).
However, stratal patterns in the greater Pennsylvanian indicate innundation of
the Midcontinent US and other shelves and basins worldwide, corresponding
to the Absaroka cratonic sequence and inferred eustatic rise associated with it
(Haq et al. 1987). The GS, also classified as a third-order cycle, exhibits an apparent tectonic signal here, but may also be the result of a superimposed eustatic influence. Experimentation with a two-dimensional stratigraphic simulation modeling has reproduced the observed succession and carbonate margin.
The backstepping and drowning of the lower shelf were modeled using a rapid
(5 to 10 m/ka) 15 m rise in relative sea level coupled with high frequency glacio-eustatic sea level oscillations of 110 m during each GSU (Watney et al.
1995). Results of modeling imply that tectonics or eustacy could explain the
observed two-dimensional geometries. However, when characterizing the
shelf in three dimensions as was done in this current investigation, reactivation of basement enters in strongly as a control. Episodic structural fragmentation of the shelf is suggested here to be as important as eustacy in developing
genetic and sequence stratigraphic models. It may be necessary to draw on
greater stratigraphic intervals and utilize larger areas to establish 3-D control
in order to develop empirical models that can better discern cause and effect
and, in turn, improve our attempts at developing predictive stratigraphic
models.
Utilization of a series of genetic units, representing coherent, temporally distinct depositional packages, reveals an evolution in the shelf configuration, each
unit analogous to a strobe, freezing the motion in that time interval. In contrast,
aggregate stratigraphic intervals that are not time-distinct will blur or remove
the effect of the changing shelf.
357
tinct genetic stratigraphic units resolves km-sized regions containing similar
sediment accumulation (sediment accumulation regions, SARs). Rhombic and
arcuate shapes of these SARs correspond closely to gravity and magnetic trends
and lineaments and known structure in the Precambrian basement.
A gamma-ray cross-section reveals that SAR boundaries affect all scales of
stratigraphic units including GS, GSU, and smaller-scale stratigraphic units and
lithofacies. In particular, SAR boundaries control the location of prominent carbonate margins consisting of single and stacked GSUs. Recurrence of carbonate
margins at the same location in different GSs suggests consistency in the site of
sediment starvation and drowning of the shelf. Moreover, the linear trends of
these boundaries and their correspondence with basement structure strongly
links them to sites of basement reactivation. Sediment accumulation regions appear to be validated from this investigation.
GSs are defined as coherent packages of related GSUs. They exhibit abrupt
transgression and more gradual regression, characterized by backstepping of
carbonate-dominated strata and lateral accretion of the same. The duration of
a GS is estimated to be approximately 1.5 Ma (Watney et al. 1995) and is of duration similar to episodic foreland basin subsidence resulting from thrusting
and downloading along collisional margins (Quinlan and Beaumont 1984).
However, stratal patterns in the greater Pennsylvanian indicate innundation of
the Midcontinent US and other shelves and basins worldwide, corresponding
to the Absaroka cratonic sequence and inferred eustatic rise associated with it
(Haq et al. 1987). The GS, also classified as a third-order cycle, exhibits an apparent tectonic signal here, but may also be the result of a superimposed eustatic influence. Experimentation with a two-dimensional stratigraphic simulation modeling has reproduced the observed succession and carbonate margin.
The backstepping and drowning of the lower shelf were modeled using a rapid
(5 to 10 m/ka) 15 m rise in relative sea level coupled with high frequency glacio-eustatic sea level oscillations of 110 m during each GSU (Watney et al.
1995). Results of modeling imply that tectonics or eustacy could explain the
observed two-dimensional geometries. However, when characterizing the
shelf in three dimensions as was done in this current investigation, reactivation of basement enters in strongly as a control. Episodic structural fragmentation of the shelf is suggested here to be as important as eustacy in developing
genetic and sequence stratigraphic models. It may be necessary to draw on
greater stratigraphic intervals and utilize larger areas to establish 3-D control
in order to develop empirical models that can better discern cause and effect
and, in turn, improve our attempts at developing predictive stratigraphic
models.
Utilization of a series of genetic units, representing coherent, temporally distinct depositional packages, reveals an evolution in the shelf configuration, each
unit analogous to a strobe, freezing the motion in that time interval. In contrast,
aggregate stratigraphic intervals that are not time-distinct will blur or remove
the effect of the changing shelf.