Page 175 - The Petroleum System From Source to Trap

9. Overburden Rock, Temperature, and H e at Flow 167

( 1 00-50 m/m.y.) to a passive margin basin (20-10
m/ m.y.). The lowest sedimentation rates (-10 m/ m.y.) sedimentary
are found in intracratonic basins such as the Michigan, rock
Illinois, and Williston basins in North America (Sleep,
1971; Schwab, 1976; Sleep et al., 1980). Strike-slip and crust
forearc basins are characterized by much higher rates
(1000-100 m/m.y.). Foreland basins experience the most sedimentation
varied sedimentation rates, but generally occupy the �
middle ground. The highest sedimentation rates are
found in areas of rapidly prograding river deltas (e.g.,
U.S. Gulf Coast basin), where sediment deposition can be P m
as much as 100G-5000 m/m.y. (Sharp and Domenico,
1976; Bethke, 1986; Bredehoeft et al., 1988). mantle
Geothermal gradients in sedimentary basins also vary Pm > Pc ) P s
widely, from as low as 1 0 " -15°C/ k m to as high as
50°-60°C/km. Part of this variation can be attributed to
differences in the background thermal state of the crust h - -
on which the basin rests. However, the thermal proper­
ties of sediments (e.g., thermal conductivity) and
Figure 9.2. Schematic illustration of isostatic subsidence
physical processes acting within basins (e.g., sedimenta­ following crustal thinning and sedimentation. Terms are as
tion and groundwater flow) are also important determi­ follows: Pc = crustal density; Ps = sediment density; P m =
nants. mantle; h d epth of compensation.
=
Why do some basins accumulate sediment much
faster than others? What controls temperature in sedi­
mentary basins and its variation between and within relative importance of tectonic subsidence and sedimen­
basins? How does sedimentation itself affect the thermal tation as driving forces for the creation of sedimentary
regime? The purpose of this chapter is to address these basins varies according to the circumstances involved.
questions by describing why and how sedimentary Major river deltas (e.g., Mississippi, Amazon, and Niger)
basins form and the physical properties and processes are primary examples in which sedimentation itself plays
that control temperature within them. a major role in forcing subsidence and increasing the
depth of a basin. On the opposite extreme, the abyssal
plains of the oceanic basins are relatively sediment
FORMATION OF SEDIMENTARY BASINS starved. They owe their existence to the cooling and
subsidence of oceanic lithosphere as it moves away from
Sedimentation and Subsidence the site of its creation at a mid-oceanic spreading ridge;
sedimentation is limited and plays an insignificant role in
A sedimentary basin is any downwarped area of the determing total subsidence.
continental or oceanic crust where sediments accumulate
and compact with burial into sedimentary rock. The Isostasy and Flexure
accumulation and removal of these rocks defines the life
cycle of a basin, from the initial event that creates the What controls the subsidence of a sedimentary basin?
basin through senescence, culmating in eventual uplift If we assume that the lithosphere has no lateral strength,
and destruction. the principle of isostasy applies. Isostasy is the funda­
A sedimentary basin forms when a topographic low is mental principle governing the development and
created in the basement rock through either tectonic evolution of topography on the earth's surface. A
subsidence or sedimentation subsidence, or both. Sedi­ succinct mathematical statement of isostasy is that
mentation subsidence can be defined as the downward density (p) integrated over an imaginary column
movement of the basement rock-sedimentary rock extending from the surface to the depth of compensation
contact in response to sediment loading (e.g., a major remains constant:
river delta), while tectonic subsidence is the subsidence of
basement rock that occurs, or would occur, in the (1)
absence of sedimentation (e.g., the deep ocean basins).
In general, both tectonic subsidence and sedimenta­ where z is depth and h the depth of compensation,
tion are necessary for the creation of a sedimentary basin. commonly taken as near the base of the lithosphere, or
Sediments accumulate only in topographic lows, thus a about 100 km (Figure 9.2). More simply stated, equation
basin must generally exist before the fill. Conversely, 1 is a mass balance equation. The total mass of material
sedimentation reinforces the tectonic subsidence that was in a column between the surface and the depth of
initiated by a basin-forming event. The load due to accu­ compensation must be constant. If the mass (or weight)
mulated sediments is capable of increasing total basin of the column increases, the column must sink, or isostat­
depth by a factor of two or three (Turcotte, 1980). The ically subside. As the column sinks, relatively high