Seismic Sequence Analysis
Exercise Four - THE SEISMIC LINE

The North-East South-West regional seismic seismic of the exercise crosses an approximately linear depression, known as the Andros Channel, had a depth of around 500 m (1500 ft) in the Late Tertiary (Eberli and Ginsburg 1997, Eberli et al 1994). This depression appears to have been filled from the Oligocene to close to the end of the Miocene by a series of enormous wedges of prograding carbonate slope sediment that extended westwards over basinal carbonates,and were capped by aggrading shelf carbonates. The timing of the fill of this depression is inferred on the basis of wells drilled on the line and to the North West. Never-the-less the exact ages of the sequences of the sedimentary section are not known (Sen and Kendall 1999).

Type I, second order seismic sequence boundaries can be identified on a seismic cross section for West Andros. These can be used as brackets to correlate enclosed third order events with an eustatic sea level curve.

The Early Neogene fill of the Straits of Andros is assumed to have had an uncomplicated and uniform tectonic setting. This simplification means that the sea level cycle chart of Haq et al. (1987), updated for the Neogene with the absolute ages provided by Berggren et al. (1995 can be matched to the interpreted seismic. This exercise asks the question can the amplitude and ages of the sea level changes as shown in the eustatic chart be used to date the sequence boundaries for the Neogene of the Straits of Andros. Does this mean that seismic sections can be dated using sequence stratigraphic geometries when the biostratigraphic data is poor?

Sequence stratigraphers interpreting seismic lines are confronted with the difficulty of determining: a) the ages of sequence boundaries interpreted on the seismic (Miall 1990); and b) the size of eustatic sea level changes associated with those (Burton et al., 1988).

You will use the principles of sequence stratigraphy to interpret a seismic cross section, that records the Neogene carbonate fill of the Straits of Andros in Great Bahamas Bank. At the heart of this study is the recognition that eustatic events are evidenced in sedimentary sections by the presence of synchronous sedimentary sequences and the unconformities that bound them (Vail et al., 1978). These eustatic events produce changes in the accommodation for sedimentary fill that have worldwide extent. Their chronostratigraphic correlation is dependent upon reliable time markers spaced sufficiently close in time to bracket the sediment packages formed in response to changes in sea level. The measurement of the amplitudes of sea level events on the sea level charts are dependent on assumptions related to the rates of subsidence and sediment accumulation for the regions in which the charts were created (Burton et al., 1988).

However there appears to be no direct method available to measure these amplitudes of sea level variation. This is because there is no datum available to measure from, particularly since the earth surface constantly moves in response to 1) sediment compaction, 2) isostatic response to sediment loads, and 3) thermal tectonic movement (Burton et al., 1988). The relative positions of sea level are thus dependent on tectonic behavior and eustatic position, and the size of either of these two variables can only be measured by assuming a model for the other's behavior.

Methods used to measure sea level indirectly have to assume models of tectonic behavior. Such methods include tide gauges, strandline position (which assume a continental relief in addition to tectonic behavior), paleobathymetry, seismic sequence onlap, stacked subsidence curves, and the matching of sequence geometries with graphical simulations (Burton et al., 1988). Despite the fact that sea level amplitudes cannot be measured independently stratigraphic predictions based on eustatic sea level curves and/or tectonic models of behavior can be reproduced and verified away from areas of interest. This is because the onlapping or downlapping of sediment geometries are dependent on rates of sedimentation, tectonic movement and sea level position. So if it is assumed that the sea level fluctuations are the same at different locations, any change in accommodation will be the product of the local tectonics and sedimentation. However, in a particular area, if rates of subsidence and of carbonate accumulation are constant for several cycles in eustatic sea level, the frequency and amplitude of the onlapping sequence geometries will be the product of the frequency and amplitude of the changes in eustatic sea level. The hypothesis you will examine is: Do the sequence geometries interpreted on seismic line match the events on the eustatic sea level curve, and then can the ages of these latter sequences have the same timing as the former. The shallow water carbonate platform of Strait of Andros in Bahamas provides a perfect opportunity to test this hypothesis.

ASSUMPTIONS
It is assumed that there was a uniform rate of subsidence and a high rate of constant sediment accumulation, for several sea level cycles. The resultant accommodation and its fill is sufficient to be recorded on the seismic section for the Neogene of the Bahamas platform. The changing onlapping position of the sequence geometries are assumed to have been produced through several cycles of sea level change and to be independent of tectonics. In the absence of absolute sea level markers for a paleoshore, it is assumed that the rate of sedimentation was sufficient to fill any shoreward accommodation to sea level. In this case the equivalent bedding plane and the shelf margin can then be used as a proxy of the sea level position. This assumption is a reasonable one for a carbonate shelf (especially in areas of high sediment production and low tectonic subsidence) and the identification of confirmatory paleobathymetric markers (Eberli et al., 1997) proves its validity. The reasonableness of this assumption can be seen in the seismic sections of the Neogene of the Bahamas. The sedimentary section developed is the product of carbonate accumulation rates high enough to fill the accommodation space up to sea level during each sea level cycle. In this case, the Neogene section is expressed by prograding clinoforms enveloped shoreward by aggrading horizontal shelfal units. In the Bahamas, this effect can be seen at both the western side of the bank and in an interior sea, the Straits of Andros (Eberli and Ginsburg 1989).

INTERPRETATION OF THE SEQUENCE STRATIGRAPHY OF THE SEISMIC LINE

Introduction to the data set
The establishment of the sequence stratigraphy of the Straits of Andros involves an interpretation of carbonate platform based on the seismic data and the limited well control from the western Great Bahamas Bank.

This Bahamian seismic data set consists of a cross-bank profile. The top 1.1 seconds (two-way travel time) is used for your study of this line from the western side of Great Bahamas Bank. Seven wells have been drilled along this seismic line; one of them, ODP site 1007, reached the base of the Neogene. Seismic profiles of the northwestern Great Bahamas Bank have been interpreted to document the lateral growth potential of isolated platforms that were welded together by margin progradation to form larger banks (Eberli and Ginsburg, 1987, 1989). The mechanism responsible for the evolution of the carbonate margin from aggradation to progradation is thought to be sediment overproduction with respect to accommodation on the platform (Hine et al., 1981, Wilber et al., 1990). Excess sediment was transported offbank and decreased the accommodation space on the marginal slope. Progradation occurred in pulses that are interpreted to be the result of third-order sea level fluctuations (Eberli and Ginsburg, 1989). The biologic and sedimentary data from the wells from the Bahamas transect have corroborated this (Eberli, et al., 1997).

Methodology for updating the sea level curve
Recent biostratigraphic studies suggest that some of the Neogene ages on the Haq et al. (1987), sea level chart are inaccurate. For the purpose of this study Sen and Kendall(1999) have updated the ages on the Late Neogene portion of this chart. The ages of the nannofossils and magnetic polarity reversal boundaries of Berggren et al. (1995), were used to update and define the timing of "sequence boundary ages" on the the Haq et al chart, curve which matched periods of maximum sea level fall. To enable comparison with the sequence boundaries identified on ODP leg 166 (which were given ages from the nannofossil zones), the ages derived from nannofossil horizons were given preference over those derived from polarity reversals. The updated "sequence boundary ages" on the Haq et al. (1987), curve are shown in drawing of the interpreted Seismic section (Figure) from the Andros Channel, Bahamas, showing the major sequence boundaries identified in the section with their interpreted ages.

The corresponding magnitudes of sea level position from Haq et al., (1987), curve were then used to draft a sea level curve (Neogene Curve (NC). The intermediate ages of sea level position between sequence boundaries were linearly interpreted. This Neogene Curve (NC) can now be used for your study.

Seismic stratigraphic interpretation: Eberli and Ginsburg (1989) previously correlated sequence boundaries on a seismic section across the Straits of Andros to "sequence boundaries" on the Haq et al. (1987), chart. In this study your will re-interpret and correlate the line with the Neogene Curve (NC) and its new sequence boundary ages. Second order Type I unconformities will be first identified on the seismic on the basis of their more extensive erosional character. Their correspondence with the "sequence boundaries" associated with the second order sea level events on the Neogene Curve (NC) should be noted. Using those as brackets, third order Type I unconformities should then identified on the seismic with their enclosed equivalent seismic sequences and correlated with the third order "sequence boundaries" on the Neogene Curve (NC). The interpretation of the seismic data should show that there the major events on the Neogene Curve (NC) have produced distinct stratigraphic signals (Figure seismic line). For instance, following the large sea level fall at 28.5 Ma, a major unconformity should be expected and so the major erosional event on the seismic can be interpreted to be equivalent to this. This unconformity separates the Upper Neogene carbonate accumulation from the Lower Tertiary and the Cretaceous. Similarly, a major fall in sea level at 11.3 Ma should also produce a corresponding major unconformity on the seismic section. The sea level should have fallen below the shelf margin, with the resulting unconformity bracketing a series of third order sea level events which date between 28.5 Ma and 11.3 Ma. While examining the geometric position of these latter sequences with respect to the shelf margin, and counting them, it may be found that more unconformities can be identified than there are events on the sea level chart. Despite these additional sequences, there should be a good correspondence of the lateral extent and thickness of the different sequences on the seismic with the amplitude and duration of the different sea level events.

Having made the sequence stratigraphic interpretation, you should ask yourself what the origin of the extra sequences is. It may be that some of these extra sequences are a result of inadvertent interpretation of the products of both low and high stand cycles of sea level as sequences. Other extra sequences could be the products of fourth order sea level events.