Sequence Stratigraphy
The entire Terry Sandstone interval, from the top to the base of the D2 Bentonite (the D2 bentonite is actually stratigraphically beneath the 'base Terry Sandstone') does not dramatically change in thickness (140 - 160ft.) across the Hambert-Aristocrat area (Fig. 8). However, the internal stratigraphy is quite complex. Applying sequence stratigraphic principles to the cores and to well log correlations, the Terry Sandstone can be subdivided into at least seven parasequences (Figs. 5, 9 and 10, labelled A-G). Individual strata within parasequences generally grade laterally seaward from blocky/thinning-upward to thickening/cleaning-upward successions (Fig. 5); this gradation is typical of shoreface parasequences (VanWagoner et al., 1990, their Fig. 8). With one exception, transgressive marine shales separate each parasequence.
Depositional history of the seven parasequences within the Hambert-Aristocrat area is undoubtedly complex. Numerous recent studies of superbly exposed outcrops of shoreface sequences, published in Van Wagoner and Bertram (1995), attest to their complexity and the numerous significant erosional surfaces that will not usually be detectable or correlatable on subsurface well logs alone. Thus, the interpretation put forth below (Fig. 8) may even be too simplistic, particularly for parasequences B - D.
Parasequence A (Figs. 5, 9 and 10) is the open shelf mudstone beneath the base of the Terry Sandstone. In much of the area, its top is erosionally truncated.
Parasequence B (Figs. 5, 9, 10, and 11) is up to 30ft. thick and overlies the basal shelf mudstone of Parasequence A. In core and on well logs, this parasequence exhibits a sharp, erosional base toward the west and northwest (paleolandward) which becomes a gradational contact toward the east and northeast (paleoseaward). There is a corresponding change in well log pattern from blocky/thinning- to thickening/cleaning-upward in the same direction. The sharp, erosional base is interpreted to have resulted from 'forced regression' (Posamentier, et. al., 1992) during sea level lowering, so it is a depositional sequence boundary. Based upon well log patterns, strata grade seaward from upper shoreface to lower shoreface/offshore. A gross interval isopach map is shown on Figure 11. Since the basal contact of this parasequence is gradational toward the northeast, isopach thicknesses could not be accurately determined there. However, the contact between the sharp-based, blocky/thinning-upward succession and the gradationally-based, thickening/cleaning-upward succession (Fig. 11; just paleoseaward of the 5ft. isopach) lies along the same trend as that discussed by Siemers and Ristow in the Antelope-LaPoudre area (1986).
Parasequence C (Figs. 5, 9, 10, and 12) is 15-35ft. thick. The base of this (and overlying) parasequences is marked by a distinct shale marker on the gamma-ray log interpreted to be a transgressive marine shale. A gross interval isopach map reveals complex stratal geometry in the paleolandward direction, with interval thicks oriented both northwesterly and northeasterly; corresponding well log patterns are predominantly blocky/thinning-upward. Toward the northeast, a clear northwesterly isopach trend prevails which corresponds to a thickening/cleaning-upward log pattern.
Parasequence D (Figs. 5, 9, 10, and 13) is 15-35ft. thick. A gross interval isopach map shows a similar geometry to that of the underlying parasequence, with mutually perpendicular thickness orientations in the paleolandward position corresponding to a blocky/fining- upward log pattern, and a northwesterly thickness orientation in the paleoseaward direction which corresponds to a thickening/cleaning-upward log pattern. The position of the boundary between wells with the two log patterns, relative to the boundary position within underlying Parasequence C, varies from somewhat paleolandward to somewhat paleoseaward.
Parasequence E (Figs. 5, 9, 10, and 14) is 15-30ft. thick. A gross interval isopach map shows that within the mapped area, there is a predominant northwestly isopach orientation which corresponds to a thickening/cleaning-upward log pattern, except along the west-southwest edge of the area, where a blocky/thinning-upward log pattern is present, and to the northeast, where a north-south thickness orientation dominates. Toward the southwest, the boundary between blocky/thinning- and thickening/cleaning-upward patterns is considerably more paleolandward than is the case with underlying parasequences.
Parasequence F (Figs. 5, 9, 10, and 15) is 10-25ft. thick. A gross interval isopach map reveals a dominant northwesterly orientation. Over most of the mapped area, this interval exhibits a /thickening/cleaning-upward log pattern except in the southwest corner where the blocky/thinning-upward log pattern prevails. The boundary between blocky/thinning- and thickening/cleaning-upward patterns is slightly more paleolandward than is the case for parasequence E.
Parasequence G (Figs. 5, 9, 10, and 16) is 15-25ft. thick. A gross interval isopach map shows an overall northwesterly thickness orientation across the entire area. Well logs of this interval exhibit a thickening/cleaning-upward log pattern over the entire area.
The base of Parasequence B is a sequence boundary (Figs. 6a and 10). This boundary is sharp-based toward the west and southwest where there has been erosion into underlying shelf shale, but is gradational and conformable toward the east and northeast (Fig. 11). Maximum lowering of relative sea level at this time resulted in the farthest seaward extent of any of the six parasequences which overlie the boundary. Each succeeding parasequence records a period of increase in rate of rise of relative sea level, followed by relative stillstand (i.e. reduction in rate of sea level rise or rate of sediment supply exceeded/equaled the rise of sea level; Kamola and VanWagoner, 1995) (Fig. 11). The backstepping nature of Parasequences B-G, as determined by successively paleolandward positions of boundaries between the blocky/thinning- and thickening/cleaning-upward strata (compare Figs. 11, 12, 13, 14, 15, and 16) indicate the parasequences record periodic stillstands within an overall transgressive systems tract. (Fig. 11).
Most of the parasequences, particularly C and D (Figs. 12 and 13), show gross interval isopach trends and corresponding well log patterns which suggest two distinctly different depositional geometries. Upper shoreface strata in the paleolandward direction exhibit complex geometries and northeasterly to northwesterly trends. Lower shoreface/offshore strata in the paleoseaward direction are characterized by less complexity and more uniform northwesterly trend. These trends reflect variations in marine reworking.
