Universidade Fernando Pessoa

Porto, Portugal

Basic Principles in Foredeep & Foldbelt Basins

III- Tectonic-Sedimentary Units

Theoretically, several tectonic-sedimentary units can be considered in foredeep basins. In the example illustrated in the fig. 1, without taking into account the folded belt on the left, which has induced the flexural subsidence that characterizes the foredeep basin, it is easy to recognize the stacking of five different sedimentary basins. From bottom to top:

a) The substratum

The substratum is often a folded belt composed by sediments belonging to a previous continental encroachment cycle. However, in certain geological settings, it can be a metamorphic or a granitic craton, or an oceanic crust.

Fig. 1-  In this sketch several sedimentary basins are stacked: (i) A substratum, which very often is composed of a fold-belt, (ii) three rift-type basins, (iii) a divergent margin, (iv) a foredeep basin and (v) a transported basin (“piggy-back). The divergent margin is bounded by two unconformities. The lower one emphasizes an uplift of the rift-type basins and the upper one underlies the beginning of the flexural subsidence (onset of foredeep basin).

b) Three Rift-type basins

The lengthening of the lithosphere, and particularly the continental crust create these sedimentary rift-type basins.

When lengthening is big enough, it can breakup the lithosphere individualizing two lithospheric plates and the associated divergent margins.

c) A divergent continental marginal (4)

The divergent margin is associated with the thermal subsidence following the breakup of the lithosphere. Generally, Atlantic-type divergent margins are composed by two stratigraphic phases. At the base: a transgressive or backstepping phase, with a global retrogradational geometry and, at the top, a regressive or forestepping phase, which has a strong progradational geometry.

(4) The presence of rift-type basins and particularly divergent margins is quite frequent when the foredeep results from the closing of an ocean. Depending on the geological setting, the divergent margins can be replaced by back-arc basin, Mediterranean basin, etc.

d) A foredeep basin

The foredeep basin is associated with a flexural subsidence (5). It is located outward of the frontal thrust. Indeed, as the orogenic system progresses toward the divergent margin (landward), different foredeep basins will be stacked. In fig. 1, for simplicity, just one foredeep basin was drawn.

(5) We will study mainly foredeep basin created by A-type subductions (Ampferer-type). Those associated with B-type subductions (Benioff-type) have not significant hydrocarbon potential (Ex: Colombia offshore.)

e) A “Piggy-back” basin

“Piggy back” basins are localized inward of the fontal thrusts. In an orogenic belt in sequence, where the age of the thrusts is younger outward of the fold belt, “Piggy-back” basins are coeval of the emplacement of the frontal ramp and associated foredeep basin.

These different tectonic-sedimentary units are bounded by unconformities more or less tectonically enhanced. Their origins are quite different as illustrated in the geological model idealized by A. Bally (fig. 2).

Fig. 2- This sketch illustrates features that are common to many foredeep basins. Four basically different unconformity types are recognized: (a) Pre-rift unconformity, (b) Breakup unconformity, (c) Basal foredeep unconformity, and (d) Shelfal unconformities. However, each foredeep has its own individual development, which is accentuated by one or another feature of this idealized diagram.

The four main discordances (unconformities) found in the foredeep basin illustrated on fig. 2 are:

a) The pre-rifting discordances (a)
           
They mark the onset of the lengthening of the lithosphere, which can end with creation of a divergent margin.

b) The breakup unconformity (b)

This unconformity marks the end of the rifting phase. It announces the onset of the sea floor spreading and the marine transgression. This unconformity predates the creation of foredeep basin. Within the pre-foredeep stratigraphic sections, there are numerous unconformities associated with relative sea level falls with a predominant eustatic component.

c) The basal unconformity (c)

In the proximal area, i.e., near the fold belt, onlapping of foredeep deep marine sediments against tilted margin sediments characterize this unconformity. In the distal area, this unconformity is mainly characterized by toplapping of the shallow water sediments of the margin (6). Actually, in the distal area, the progradation of the foredeep sediments, preserving the erosional surface is not often very subtle. The angle of the slope is too small.

(6)  Toplaps are the consequence of the outward migration of the bulge anomaly

It must be noticed that very often, immediately overlying this unconformity, there is a complex downlap surface marking the distal terminations of the progradation of the foredeep sediments.

d) The unconformities within of the foredeep basin (d)

The majority of these unconformities are associated with relative sea level falls in which the eustatic factor is predominant. When, often, these unconformities erode the anticlines synchronous with the sedimentation, they are locally tectonically enhanced.

Fig. 3- On this seismic line, it is easy to recognize the main charcateristics of a foredeep basin, which overly a divergent margin, which in its turn overlies several rift-type basins. The four basically different unconformity types: (a) Pre-rift unconformity, (b) Breakup unconformity, (c) Basal foredeep unconformitiy, and (d) Shelfal unconformities are also readily recognized.

The origin of these unconformities raises important problems in Sequential Stratigraphy (see short-course: "Seismic-Sequential Interpretation") and in the hierarchy of the stratigraphic cycles. Actually, for a certain number of discordances, as those mentioned below, the eustatic component seems insignificant, i.e. eustacy seems not to be the major geologic cause of the unconformities:

- The unconformities characterized by erosion and truncation of the tilted fault blocks.

- The unconformities induced by deepwater currents on the slope.

- The unconformities associated with canyons on the divergent margins.

- The unconformities seen in on shallow water environments induced by geostrophic currents (7).

(7) Geostrophic currents are atmospheric or oceanic currents in which the force of horizontal pressure is balanced (equal, but opposite) by the Coriolis force. They are not either accelerated or affected by friction.

- The unconformities associated with starved basins.

- The unconformities underlying the onset of seafloor spreading.

- The breakup unconformities.

- The basal foredeep unconformities.

- The unconformities associated with the relaxation of the foredeep basins.

- The unconformities associated with salt flowages, etc.

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