Continental Rifted Margins 1. Gwenn Peron-Pinvidic. Читать онлайн. Newlib. NEWLIB.NET

Автор: Gwenn Peron-Pinvidic
Издательство: John Wiley & Sons Limited
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Жанр произведения: Физика
Год издания: 0
isbn: 9781119986911
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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.

       – 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.

      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 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).

Schematic illustration of the definitions of sedimentary subdivision into pre-, syn- and post-tectonics.

Schematic illustration of summarizing the major structural and stratigraphic characteristics. Schematic illustration of the tectono-sedimentary evolution of a normal fault array.