Page 177 - The Petroleum System From Source to Trap

9. Overburden Rock, Temperature, and Heat Flow 169

McKenzie's model is often applied (and misapplied) America initially underwent fault-controlled mechanical
to estimate the timing of hydrocarbon generation. The subsidence in response to rifting. The initial phase of
usual procedure is to ''backstrip" sedimentary basin fill basin formation was followed by thermal subsidence
for the purpose of separating tectonic subsidence from and subsidence due to the isostatically uncompensated
the total subsidence. This is done by applying the mass of a cooled igneous intrusion.
principle of isostasy and compensating for factors such Although the subsidence history of intracratonic
as sediment compaction and changes in sea level basins is apparently consistent with some type of thermal
(Steckler and Watts, 1978; Sclater and Christie, 1980; mechanism, the exact nature of the initial thermal event,
Sclater et al., 1980). The estimated tectonic subsidence its subsequent evolution, and the role of other factors in
curve is then compared to McKenzie's theoretical predic­ basin genesis and development are apparently not well
tions and a ''best" value for the stretching factor � found. understood at the present time.
Once � is known, heat flow can be estimated, tempera­
ture calculated, and source rock maturity predicted Foreland Basins
(provided that the location of the source rock in the basin
fill is known). This is a straightforward approach, but Foreland basins (Beaumont, 1981) are asymmetric,
there are many ancillary determinants that must also be wedge-shaped accumulations of sedimentary rock that
taken into consideration if meaningful estimates of the form adjacent to fold-thrust belts. Migration of the
thermal history are to be made. These include the fold-thrust sheet loads the lithosphere, causing isostatic
depression of heat flow by sedimentation (De subsidence underneath the core of the orogen and
Bremaecker, 1983), the thermal conductivity of rocks flexural downwarping in the adjacent foreland. The
within the basin (Blackwell and Steele, 1989), the surface foredeep that forms next to the orogenic belt rapidly fills
temperature, and the possible influence of groundwater with sediment eroded from the adjacent mountains.
flow. The relative importance of these intrabasin factors Sedimentation amplifies flexural subsidence, and a
grows with passing time as the influence of the initial foreland basin is formed (Figure 9.1).
basin-forming event wanes.
The foreland basin process continues until the forces
driving uplift and orogeny cease. Erosion then
Intracratonic Basins dominates, reducing the weight of the mountain chain,
leading to uplift and further erosion. The life cycle of a
Intracratonic, or platform, basins form on continental
interiors (e.g., the Michigan, Dlinois, and Williston basins foreland basin is thus typically one of fairly rapid burial
and subsidence followed by a much longer period of
of North America; Figure 9.1). They are typically a few uplift and erosion. Most source rocks buried by the
hundred kilometers wide and contain a few kilometers foreland basin fill probably go through a relatively short
of flat-lying sedimentary rocks recording continuous
subsidence and sediment deposition over periods of time heating and maturation phase, followed by a longer
greater than 100 m.y. (Sleep et al., 1980). Sleep (1971) was cooling phase.
the first to note that the subsidence of these basins was, Thermal events play a minor role in the formation of
like oceanic basins, proportional to the square root of foreland basins. However, the thermal state of the lithos­
time, with a time constant of about 50 m.y. This led to phere influences its flexural strength, thereby exerting an
speculation that the formation of these basins, like rift indirect control on the structural evolution of foreland
basins, was controlled by some type of heating or basins (Watts et al., 1982).
thermal event followed by thermal contraction (Sleep,
1
1971; Sleep and Snell, 9 76; Ahern and Mrkvicka, 1984; Other T yp es of Basins
Nunn et al., 1984; Klein and Hsui, 1987).
For an intracratonic basin to be formed by thermal Many other types of basins can be defined; these type
contraction, isostasy requires that a considerable amount are potentially as numerous as the heterogeneous crust
of crustal erosion occur during the initial heating, uplift, of the earth. Some of these include strike-slip, forearc,
and thermal expansion phase. For example, if the basin and backarc (Figure 9.1). Strike-slip or pull-apart basins
fill is 3 km deep, it would be necessary to first remove are formed by lateral movement along transform faults,
about 1 km of the continental crust through erosion. literally pulling the crust apart and creating a void that
However, in many instances, there is little evidence that fills with sediment (e.g., the Los Angeles basin) (Turcotte
this type of dramatic erosion ever occurred (Sleep et al., and Ahern, 1977; Turcotte and McAdoo, 1979). Backarc
1980). Recognition of this problem has led to the and forearc basins form in back of and in front of
proposal of several alternative hypotheses. These include volcanic arcs, respectively, near subduction zones.
(1) an increase in density of the crust due to one or more Backarc basins may form from active seafloor spreading
phase transitions, (2) rifting, (3) mechanical subsidence and riftg, in which case they exhibit high heat flow. In
caused by an isostatically uncompensated excess mass of other cases, backarc basins are apparently passive
igneous intrusions, (4) tectonic reactivation along older features that may merely represent trapped segments of
structures, or (5) some combination of these or other old oceanic crust. Forearc basins are the result of
theories (see review by Klein, 1991). For example, Klein sediments filling the topographic low created by
(1991) suggests that intracratonic basins in North subduction.