NPRA Exercise Solutions

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Analysis
Exercise Three Class Solution
This is the second of the series of exercises tied to the interpretation of seismic data and its relationship to described on the page below.

Lines 12-81
Major erosional events are seen on seismic line 12-81 where the reflector clinoforms, which have been identified by coloring the horizons, on lap onto the shelf as Yellow (11), Olive Green and Rust (14 - 15), and Purple (19) horizons. Our interpretation is that the slope of the clinoforms in this section are represented by the Torok Formation. The horizontal onlapping reflectors of the crest of the Torok Formation clinoforms represent the Nanushuk Group. The same pattern is to be seen in the other seismic lines that follow.

Line 26-74
Major erosional events are seen on the seismic line 26-74 at events Orange (7), Green and Navy Blue (14-15), and Olive Green (19).

Line 27-81
Major erosional events are seen on the seismic line 27-81 at reflectors Orange and Olive Green (11-12).

Line 37-81
Major erosional events are seen on the seismic line 37-81 at reflectors Orange and Olive Green (16-17).

Proposed Sea Level events

Sequences in the Nanushuk Group and Torok Formation clinoforms were identified on the seismic data and numbered from 5 through 24. The areal extent of the sequence boundaries were then placed in order in a spread sheet and then displayed as a chronostratigraphic chart. On the basis of the earlier work by Palmer (1983) and Bird and Molenaar (1991) that suggests that the eastern most part margin of the progradational Torok Formation and Nanushuk Group were Cenomanian in age, the major lows in sea level and unconformities were identified and were assumed to be related to the major lows on the Haq and others (1987) chart for the Cenomanian, for 98 my, 96 my and 94 my.

Evolution of Basin Margin
Mapping the position of selected Nanushuk Group and Torok Formation clinoforms observed in the seismic data set indicate the paleo-shoreline was oriented northwest southeast.

The crests of the basin margins are indicated on Figure 12 below.

Conclusions
There was extensive tectonically driven accommodation developed in the Foreland Basin. Reduced accommodation on the adjacent cratonic fragment beneath the Barrow Arch to the north was overwhelmed by the sedimentation from the folded and over thrust Brooks Range to the South and the generation of clinoforms. There was a a deepening in the foreland basin to the SW and a shallowing to the NE over the Barrow Arch.

Clinoforms can be seen becoming progressively younger to the north and east. The internal geometries of these clinoforms indicate an absence of small-scale tectonic control on the area.
The variation in the thickness and distribution of the clinoform suggest that rate of sediment supply from the Aptian through the Cenomanian was variable in the NPRA. Such variation in shape and geometries are probably related to delta switching, and migration tied to the shifting of the source areas from the SW.
Individual sequences show evidence of slumping, basin floor fans, slope fans, and onlapping high stand wedges.
Major erosional events are seen on the seismic lines. This can be observed by comparing the interpretations on lines 27-81 (events 11-12), 37-81 (16-17), 12-81 (11, 14-15, &19) and 26-74 (7, 14-15, & 19) with the biggest erosional events above surface 7 and between surfaces 11 and 12.
The biostratigraphy for the area is poor and so lithostratigraphy was used to identify the Nanusuk (sand prone unit), and the Torok (silt prone unit). On the basis of the biostratigraphy and their prograding clinoforms these lithostratigraphic units can be seen to young towards the north and east with the eastern portion of these deltaic wedges being assumed to be Cenomanian in age.
On the Haq 1987 chart for the Cenomanian/Albian boundary there were three major type 1 unconformities (112 my, 109 and 107.5 my).