Exactly when the western Algerian region was structurally partitioned to form the Reggane, Sbaa-Azzene and Timimoun basins has dramatic implications for: 1) the hydrodynamic connectivity of the region and therefore the timing for the generation and destruction of migration pathways, and 2) the timing and distribution of source rock maturation. The early geological development of western Algeria was part of a major phase of intracratonic basin formation that occurred across the entire northern African continent during the early Cambrian. Two scenarios exist for the tectonic and stratigraphic development of the region: (1) that the present structuring of the Algerian region occurred early in the deformational history of the region with only minor reactivation of fault systems during the Hercynian compressional event, and (2) the early development of the Reggane, Sbaa-Azzene, and Timimoun basins was part of a significantly larger (intracratonic) basin system and the present structuring occurred predominantly in response to Hercynian compression. A first-order structural partition, the Ougarta range, separates the Reggane from the Sbaa-Azzene basins while the Timimoun basin is separated from the Sbaa-Azzene basin by a series of high-standing basement blocks. The uplift history of the Ougarta range, as elucidated from seismic stratal patterns, kinematic and isostatic basin modeling, and preliminary apatite fission-track data, is crucial in discriminating between the above scenarios and thus the time the region was hydrodynamically dissected.
Topographic basemap with location of major western Algerian basins:
The basins of western Algeria have had a protracted life beginning as part of a regionally distributed lithospheric extensional system that commenced just after the Pan African orogeny and spans to the Present. This Cambro- Ordovician extension was likely a consequence of the topographic collapse of the thickened Pan African crust and lithosphere. Thus in the Cambro- Ordovician, the Reggane, Sbaa-Azzene and Timimoun basins represent relatively large asymmetric half-graben. The major border fault systems are those of the Reggane (northern boundary) and Timimoun (southern boundary) basins. Possible modern-day analogues of this type of topographic collapse include the Basin-and-Range Province and the present orogenic collapse of the Himalayas and Tibetan Plateau. If this model is correct, then the topographically higher and broader regions of the 10,000 km-long Alpine collision (i.e. Spain to China) will, following orogenic collapse and denudation, result in an impressive series of continental basins.
Post-extensional cooling of the northwest Algerian lithosphere induced subsidence over approx. 260 Ma, beginning in the late Cambrian and continuing through the Permian (505-245 Ma), and is consistent with rifting of relatively thick lithosphere (approx. 170 km). The regional distribution of the thermal subsidence explains the general parallel-nature of the Ordovician to Carboniferous stratigraphy observed across the region, with only minor Silurian and Devonian normal fault reactivation punctuating this otherwise monotonous and long-lived subsidence. Onlap of upper Cambrian and Ordovician sediments onto basement highs throughout the region (e.g., southern Reggane basin, Ougarta range, and Azzene high) is evidence for relative topographic relief at this time. This relief most likely represents the flexural rift flank topography developed adjacent to the various half-graben.
Surficial outcrop in the Ougarta range consists of Pan African basement, thick sections of Cambrian to Ordovician carbonates and sandstones overlain by Silurian mudstones. The thick Cambro-Ordovician sediments tend to be preserved in small sub-basins within the range itself (perhaps as rider blocks), thus accounting for their preservation. Nevertheless, the existence of Cambrian-Silurian sediments on the Ougarta range imply that its uplift necessarily post-dated the deposition of these sequences.
Reactivation of the Reggane bounding fault system during Silurian and Devonian time is evidenced by the rapid increase in subsidence magnitude and the observed onlap patterns observed within the basin. Divergence and rotation of seismic reflectors is indicative of differential subsidence and block rotation, with the intensity of the deformation correlating with the amount of rotation and divergence. Each phase of differential subsidence should be recorded by a divergent stratigraphic package that shoals upward. Because glacial-eustatic sea-level variations alone cannot explain the divergence and rotation of seismic reflectors, such stratal geometries document the onset of discrete rifting events. It is because of these renewed rifting events that the Silurian and Devonian sections thin southward across the Sbaa-Azzene basin by onlap onto the uplifted footwall. In this case, the footwall block is in fact the Ougarta range. As the Ougarta range and the southern Sbaa-Azzene basin were in an updip position with respect to the Timimoun basin depocenter during this time, the footwall block represents a prime potential structural/stratigraphic trap for the migration of hydrocarbons.
These observations are consistent with the Ougarta range being a relatively high-standing block with respect to the rift basin depocenters for most of its existence, and was uplifted in response to episodes of Silurian and middle/late Devonian rifting within the northern Reggane basin. Contemporaneous with the reactivation and uplift of the Ougarta range and Sbaa-Azzene basin, deep water carbonates and shales were accumulating in the Reggane and Timimoun basins. These rifting events help explain the increased subsidence in one portion of the basin (e.g., the Reggane basin) while other regions are experiencing uplift. Our work in other actively deforming regions (e.g., Jeanne d'Arc basin) indicates that stratal discontinuities need not simply be the result of relative sea-level changes. For example, the formation of discontinuities in tectonically active basins, such as graben and foreland basins, records subsidence in one portion of the basin that may be accompanied by contemporaneous uplift elsewhere. That is, the basin may simultaneously experience both an increase and a decrease in the rate of relative sea-level change. As a result, unconformities can pass laterally from local subaerial unconformities into marine onlap surfaces.
By the end of the Paleozoic, the entire region was uplifted as part of a major northwest-southeast compression, the Hercynian Orogeny. The main effect of Hercynian compression in the study area was to locally reactivate earlier normal faults, produce small folds and associated foreland basins, uplift further the Ougarta range thereby completing the isolation of the Reggane, Sbaa-Azzene, and Timimoun basins. The amount of shortening is controversial, although we will demonstrate using Quantitative Basin Analysis that for these basins, the total shortening was relatively minor (i.e., heave of kilometers). We estimate that the regional denudation of the region to be 200-400 meters, with a maximum of 1.5-2 km of erosional truncation across the Ougarta block. Continued thermal subsidence since the formation of the Hercynian unconformity has deformed the erosional surface to produce a regional topographic low within the central Timimoun basin.
The breakup of Pangea lead to the establishment of a rift system between eastern North America and northwest Africa that ultimately resulted in the formation of Atlantic margins. The northern Timimoun basin is located landward of the Triassic passive margin, the hinge zone probably occurring in the vicinity of the present-day Atlas mountains. The northern boundary of the Timimoun basin was incorporated into the 2.0-2.5 km rift flank topography of the Triassic rift basin. Subsequent erosion of this rift flank topography truncated to at least the Silurian-Carboniferous section, with the amount of erosional truncation increasing northward towards the hinge zone. Amalgamation of the Hercynian and Triassic unconformities tends to increase the duration of the hiatus. Post-Triassic thermal subsidence induced subsidence across the northern Timimoun basin. Lower Jurassic and younger coastal plain and shelf sediments of the newly formed Atlantic margin unconformably overly the truncated and rotated sediments of the northern Timimoun basin. This Mesozoic sediments and record the flexural downwarping of the northern Timimoun basin in response to post-Triassic thermal subsidence and sediment loading across the Atlantic passive margin creating an asymmetric wedge of sediment of up to 2 km thick.
