236
6 DEPOSITIONAL SYSTEMS
Arctic to the Gulf of Mexico. In response to the uplift of the Laramide orogeny, vast
quantities of detritus were brought into this sea-way from the west. This was deposited
in a wide range of environments ranging from the piedmont fanglomerates of the Mesaverde Group to the offshore marine shales of the Pierre and Lewis formations. Transitional shoreline deposits were laid down in a great diversity of environments. Both
linear shoreline and lobate shoreline deposits have been recognized. The deltaic deposits seem to have been deposited in marine dominant deltas analogous to the Niger
and the Nile. Notable accounts of these beds have been given by Weimer (1970, 1992),
Asquith (1970), van de Graaff (1972), McCubbin (1982), Devine (1991), and Martinsen
et al. (1993).
Specific examples of Rocky Mountain Cretaceous barrier island sand bodies include
the Eagle and Muddy sandstones of Montana (Shelton, 1965; Berg and Davies, 1968),
and the Bisti bar oil field of New Mexico (Sabins, 1963, 1972).
These examples, taken from both surface and subsurface studies, have a number of
common features. They all tend to show an upward-coarsening grain-size profile. The
bases of the sand bodies are transitional with open marine shales. The tops of the sands
are abrupt, often marked by a break in sedimentation, and overlain by marine or nonmarine shales. Internally these sand bodies are generally bioturbated in their lower
parts and massive or subhorizontally bedded toward the top. The most diagnostic feature of all, however, is that these sand bodies are linear shoe-strings running parallel to
the local shoreline.
Modern coastal geomorphologists rigorously define coastal sand bodies into barrier
islands, offshore bars, spits, and tombolos. It is easy to make these distinctions of modern sand bodies. They are not easy to make of ancient sand bodies because their spatial
relationships are seldom sufficiently clear. Most geologists are content to label a particular unit as a bar sand. Additional refinements of its degree of exposure or geomorphology are largely academic.
Lateral progradation of barrier bars can generate not just a shoe-string, but a sheet
sand body. This requires a critical balance between sediment input and subsidence.
Nevertheless, examples of barrier sheet sands have been described, notably from the
Cretaceous of the Rocky Mountain foothills and from the Gulf Coast of Mexico Tertiary. Descriptions of Rocky Mountain bar sheet sands have been given by Hollenshead
and Pritchard (1961), Weimer (1961), and Asquith (1970). Descriptions of the Gulf Coast
sand bodies have been given by Burke (1958) and Boyd and Dyer (1966). These studies
show that the sand sheets occur not just as regressive sand sheets, which pass down into
marine and up into nonmarine facies, but also as trangressive sands which pass up into
marine facies. The transgressive sand bodies tend to be less well developed, however,
and the transgressions are more often represented by surfaces of erosion than by depositional units. Close examination of the transgressive sand bodies shows that they are
actually composed of a series of upward-coarsening shoe-string units, which are vertically arranged en echelon (e.g., Asquith, 1970, Fig. 34). This implies that deposition of
sand during a marine trangression actually takes place only during still-stands of the
sea when bars may prograde seaward. In sequence stratigraphic terminology (discussed
later in this chapter) barrier bar sands are part of the transgressive systems tract in a
third-order sequence cycle.
Barrier bar sands are often stacked in a series of transgressive and regressive cycles
(Fig. 6.44). Unlike the deltaic sedimentary model, the barrier model lacks a built-in
6 DEPOSITIONAL SYSTEMS
Arctic to the Gulf of Mexico. In response to the uplift of the Laramide orogeny, vast
quantities of detritus were brought into this sea-way from the west. This was deposited
in a wide range of environments ranging from the piedmont fanglomerates of the Mesaverde Group to the offshore marine shales of the Pierre and Lewis formations. Transitional shoreline deposits were laid down in a great diversity of environments. Both
linear shoreline and lobate shoreline deposits have been recognized. The deltaic deposits seem to have been deposited in marine dominant deltas analogous to the Niger
and the Nile. Notable accounts of these beds have been given by Weimer (1970, 1992),
Asquith (1970), van de Graaff (1972), McCubbin (1982), Devine (1991), and Martinsen
et al. (1993).
Specific examples of Rocky Mountain Cretaceous barrier island sand bodies include
the Eagle and Muddy sandstones of Montana (Shelton, 1965; Berg and Davies, 1968),
and the Bisti bar oil field of New Mexico (Sabins, 1963, 1972).
These examples, taken from both surface and subsurface studies, have a number of
common features. They all tend to show an upward-coarsening grain-size profile. The
bases of the sand bodies are transitional with open marine shales. The tops of the sands
are abrupt, often marked by a break in sedimentation, and overlain by marine or nonmarine shales. Internally these sand bodies are generally bioturbated in their lower
parts and massive or subhorizontally bedded toward the top. The most diagnostic feature of all, however, is that these sand bodies are linear shoe-strings running parallel to
the local shoreline.
Modern coastal geomorphologists rigorously define coastal sand bodies into barrier
islands, offshore bars, spits, and tombolos. It is easy to make these distinctions of modern sand bodies. They are not easy to make of ancient sand bodies because their spatial
relationships are seldom sufficiently clear. Most geologists are content to label a particular unit as a bar sand. Additional refinements of its degree of exposure or geomorphology are largely academic.
Lateral progradation of barrier bars can generate not just a shoe-string, but a sheet
sand body. This requires a critical balance between sediment input and subsidence.
Nevertheless, examples of barrier sheet sands have been described, notably from the
Cretaceous of the Rocky Mountain foothills and from the Gulf Coast of Mexico Tertiary. Descriptions of Rocky Mountain bar sheet sands have been given by Hollenshead
and Pritchard (1961), Weimer (1961), and Asquith (1970). Descriptions of the Gulf Coast
sand bodies have been given by Burke (1958) and Boyd and Dyer (1966). These studies
show that the sand sheets occur not just as regressive sand sheets, which pass down into
marine and up into nonmarine facies, but also as trangressive sands which pass up into
marine facies. The transgressive sand bodies tend to be less well developed, however,
and the transgressions are more often represented by surfaces of erosion than by depositional units. Close examination of the transgressive sand bodies shows that they are
actually composed of a series of upward-coarsening shoe-string units, which are vertically arranged en echelon (e.g., Asquith, 1970, Fig. 34). This implies that deposition of
sand during a marine trangression actually takes place only during still-stands of the
sea when bars may prograde seaward. In sequence stratigraphic terminology (discussed
later in this chapter) barrier bar sands are part of the transgressive systems tract in a
third-order sequence cycle.
Barrier bar sands are often stacked in a series of transgressive and regressive cycles
(Fig. 6.44). Unlike the deltaic sedimentary model, the barrier model lacks a built-in