Читать книгу Continental Rifted Margins 1 - Gwenn Peron-Pinvidic - Страница 42

Sedimentation, although always present in rift basins, is extremely variable from one system to the other. It mainly depends on the surrounding geography (morphology) and lithological and climatic parameters. For detailed information regarding the processes related to these parameters, the reader is referred to contributions specifically dedicated to sedimentary processes. Here we will only list the basic architectural definitions that are considered pertinent when studying the rift space and temporal evolution.

Traditionally, sedimentary sequences of rift basins are subdivided into three categories: pre-, syn- and post-rifts (Figure 1.29):

 – Pre-rift sediments are deposited before the rifting phase. They are considered to be mechanically part of the basement and they are deformed and displaced together during the subsequent rift-related tectonic phases.

 – Syn-rift sediments are deposited during extensional activity. These sediments are captured in basins created by the movement of the basement blocks. If the relationship between the activity of the faults and the sediment supply correlates well, the syn-rift sediments are deposited in a fan geometry created by the rotation and the subsidence of the blocks.

 – Post-rift sediments are deposited on the margin, after extensional activity has ceased, draping and smoothing the topography.

In theory, discontinuities mark the separation between pre-, syn- and post-rift sediments. For instance, the breakup unconformity is generated by an uplift event that is related to the final rupture of the continental lithosphere and emplacement of the first oceanic crust. The uplift generates a level of erosion in the stratigraphic record and/or an absence of record.

These basic definitions are extremely useful and often allow for a clear and effective subdivision of the rift history. However, they have been deeply challenged over the last few decades. Based on recently available high-resolution geophysical datasets, it appears that these strict definitions may not encompass the multiphase structural evolution of some extensional systems, and/or the complex geometries of certain faults: for instance, the tectonic movement accommodated by a high-angle normal fault (top left in Figure 1.29) is very different from the displacement generated on a low-angle detachment fault (bottom case in Figure 1.29). The sedimentary geometries that are potentially generated in these two opposing cases will contain very different characteristics. Additionally, it is now recognized that most rift systems evolve through distinct phases of successive tectonic activity that often propagate oceanwards. The result of these multiple phases is that a sedimentary unit that is interpreted as syn-rift in one basin may correspond to a pre-rift or a post-rift setting in an adjacent basin. The terms pre-, syn- and post-rifts are therefore misleading when used at margin-scale studies. Therefore, new definitions have been proposed to classify sediments in rift systems: pre-tectonic, syn-tectonic and post-tectonic sediments (see the discussion in Chapter 2 for further explanation). These distinctions allow sediments to be classified depending on whether they have been deposited before, during or after phases of tectonic activity. These new terms reflect the multiphase evolution characteristic of most extensional systems better than pre-rift, syn-rift and post-rift sediments.

The processes governing the production, transport and deposition of sediments in extensional areas are complex. Many parameters can influence the sediments’ lifecycle and are, moreover, often inter-dependent. These influencers can be categorized into two categories: non-tectonic, such as climate conditions (arid, tropical), source rock composition, position relative to lake or sea level and eustatic fluctuations (Prosser 1993); and tectonic, such as the type of sedimentary basin, characteristics of the multiphase activity of the fault(s) bounding the basin and subsidence. All of these tectonic influences directly impact the source location, transport route and deposition location.

The fault type and fault growth pattern directly control the subsidence rate, depocenter geometry and position, and facies distribution in the basin (see the schematic comparison in Figure 1.30). In a half-graben with HANFs, the basin’s asymmetry naturally imposes a strong influence on the syn-tectonic deposition patterns, and the footwall uplift, which can be very pronounced in some settings, strongly affects the drainage system (Leeder and Jackson 1993). The footwall provides coarse materials in the form of alluvial fans, whereas the material from the hanging wall is finer-grained on average (e.g. Gawthorpe and Leeder 2000) (Figure 1.31). In another case, basins sitting above low-angle detachment faults mostly collect fault products that are derived from the footwall breakaway, and the sedimentary geometries are very different from more standard half-graben-type basins (Figure 1.30). Therefore, special attention should be paid to the structural context in which the basin has been formed when studying the architecture and geometric relationship of a sedimentary basin. Additionally, fault propagation, growth, death and linkage are important key parameters controlling the final sedimentary architecture. For example, the presence of relay ramps between basins will cause deviations in the source and routing system.

The sediment composition also changes during rifting evolution because of changes in the source and routing system, as seen in changes from continental to marine environments. In general, most of the sedimentation histories begin with fluvial, lacustrine and subaerial deposits and evolve with time to more marine conditions ranging from near shore to distal offshore conditions.

The detailed study of sedimentary successions related to the geologic time scale is called stratigraphy. Sequence stratigraphy addresses how depositional architecture forms in response to (changes in) accommodation and sediment supply. Readers interested in these are referred to contributions specifically dedicated to these topics.

Further reading.– The above descriptions are abbreviated and often simplified. If interested in reading and learning further, the reader is referred to the following list of publications and references:

 – General: (Leeder and Gawthorpe 1987; Prosser 1993; Gawthorpe and Leeder 2000; Wilson et al. 2001; Masini et al. 2013; Ribes et al. 2019).

Figure 1.29. Schematic representation of the definitions of sedimentary subdivision into pre-, syn- and post-tectonics, illustrated in different extensional contexts

Figure 1.30. Schematic diagrams summarizing the major structural and stratigraphic characteristics of (a) a high-angle normal fault bounding a half-graben-type basin inspired by the Il Motto half-graben in the Ortler Nappe (Northern Adriatic proximal margin) and (b) an extensional detachment fault system with extensional allochthonous blocks and a supra-detachment basin inspired by the Samedan basin in the Err nappe (Northern Adriatic margin) (source: from Ribes et al. 2019, modified after Wilson et al. 2001 and Masini et al. 2011)

Figure 1.31. Series of block diagrams illustrating the tectono-sedimentary evolution of a normal fault array (continental environments) (source: modified from Gawthorpe and Leeder 2000)

CONTINUATION OF CAPTION FOR FIGURE 1.31.– a) Initiation stage; b) interaction and linkage stage and c) through-going fault zone stage. The diagrams show how sediment transport pathways are influenced by fault geometry and activity and by antecedent drainage networks that are locally modified by surface topography. See Gawthorpe and Leeder (2000) for detailed explanations of the model that includes five more stages not shown here.