Bahamas Solution

Western Geophysical Line NW of Andros Island on South central Great Bahamas Bank (NASA image)
Final Report, Analysis and Solutions
The laterally stacked sequences of a seismic section are the product of tectonic movement and carbonate accumulation and also of sea level changes whose signal can be identified by making sequence stratigraphic interpretations (Vail et al., 1988). As Eberli et al (1994) indicated, a sedimentary simulation can successfully reproduce the geometries seen on a seismic line. The seismic data that documents the sedimentary fill of the Straits of Andros exhibits a series of onlapping and downlapping edges with various angles of slope. It would appear that different basin depths affected the timing and extent of progradation. They also show that there was a close balance between aggradation and progradation, and that small changes in the rate of relative sea level movement and/or carbonate accumulation rates caused immediate switches from aggradation to progradation in the margin. progradation took place after the space that existed on the upper slope had been reduced and the sediment-transported offbank was able to fill the remaining space downslope so extending the platform margin farther basinward. In particular, progradation was triggered by sea level drops that shifted sediment production and accumulation to the margin slope. carbonate production rates similar to modern rates were required to produce the sediment necessary for progradation, which suggests that carbonate production has been consistently high since the early Tertiary. At the same time repeated exposure and erosion have reduced the overall preservation and decreased the overall accumulation rate, while progradation occurred in pulses which are recorded on the seismic lines and are confirmed by the simulation to be a succession of prograding and sigmoidal sequences, with each sigmoid clinoform apparently having formed as a result of a single cycle of sea level fall and rise (Eberli and Ginsburg, 1989) (See attached two movies that capture the Sedpak simulation).

Neogene Sedimentary Carbonate Fill of Western Andros Channel, Bahamas  
Neogene Sedimentary Carbonate Fill of Western Andros Channel, Bahamas displayed as Sequences.
 
The offlapping complex of reefal carbonates covered by calcareous sand. Eberli and Ginsburg (1989) thought that during the transgressive stages, marginal reefs were established and then buried during the subsequent highstand, when abundaformformnt sediment was produced on the flooded bank. Their interpretation was based on findings from the leeside of the modern bank where early Holocene reefs are covered by offbank transported sand (Hine et al., 1981).

The two 1990 core borings on the western margin of the Great Bahamas Bank have confirmed this interpretation. Interestingly, the seismic section suggests progradation of the bank margin continued at sea level lows even though the platform was exposed. It would appear that either in situ accumulation remained high at sea level lows or that the platform sediments were eroded. For a carbonate shelf setting with a low rate of subsidence and a high rate of sedimentation, very clear stratigraphic signals should be produced by eustatic sea level changes. This requires that the rate of carbonate sedimentation be such that the accommodation was filled to sea level, suggesting that the sediment surface on the shelf can be taken as a proxy of this sea level. For such a case, when 1) the rates of subsidence and carbonate accumulation were constant during several sea level cycle; and 2) there was a match in the frequency and amplitude of the onlapping geometries of seismic and simulation, then it can be assumed that the frequency and amplitude of eustatic events of the input curve, the corrected Neogene Curve (NC) used for this study, is close to reality. This match in the seismic and the simulation geometries established that ustatic sea level charts can be used to date seismic sequence boundaries, as was the case of the Neogene carbonate platform of the of Straits of Andros in Bahamas. It also shows that the sea level curve can be used to test seismic interpretations and update the ages of sequence boundaries when biostratigraphic data is poor. Try to match your interpretation with results of ODP leg 166 Preliminary reports.

The ages of the OPD SSBs are based on biomarker species horizons and the use of interpolated sedimentation rates between them. One might argue that the mismatch is because the the Haq et chartal chart is inaccurate or that some events are not as global as was thought. On the other hand the mismatch may be produced by a tectonic signal mixed with a global sea level signal, producing leads and lags in the formation of sequence boundaries. However, the fact is that your interpretation and the other from ODP, should match closely, and in this case this is a compelling argument for this kind of interpretation for dating similar sections where biostratigraphy is poor.

Click here for a solution and match the output with your interpretation. For a fuller description of this solution, click on Eberli et al (1994), Kendall et al (1998), and Sen and Kendall (1999).

To gain access to the complete image containing the interpreted seismic section of the exercise, click on the image below to download it as one image or as a series of images (each divided image can be printed on a regular 11" X 8.5" paper).
Each prograding sequence was up to 500 m thick and probably consisted of an


 

Monday, July 25, 2016
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