INVITED EDITORIAL Processes and controls in the stratigraphic development of extensional basins

S. Gupta, P. Cowie
2000 Basin Research  
INTRODUCTION The sedimentary rocks that are preserved as the in®ll to continental extensional basins provide a tantalizing record of the evolution of coupled geomorphic±sedimentary systems in response to rifting. This record is tantalizing because, even though the stratigraphic record can arguably be considered to provide the only complete record of the effects of external forcing on basin evolution, how the sedimentary effects of this forcing come to be preserved in the stratigraphic record is
more » ... not well understood. The stratigraphy rarely enables us to make direct links with forcing factors because independent evidence of the latter is generally no longer preserved. Thus our task of extracting the in¯uence of factors such as variations in tectonic subsidence and uplift, source-area erosion, sea-level change, climate, etc., from the stratigraphic record is daunting to say the least. Indeed, some would argue that the problem is largely intractable. Despite this, sedimentary geologists and stratigraphers are increasingly armed with a whole new range of tools that enable them to tease more information out of the apparently unyielding rocks. These tools include new dating techniques, novel geochemical methods and numerical modelling approaches. However, more importantly, we are currently seeing a revolutionary transition in our approach to the subject. Sedimentary geologists are no longer merely content to describe, document and speculate on the rocks they study, but are seeking to understand in more depth (and with greater quanti®cation) the processes that govern their organization. Our aim in this special issue is to examine some of the emerging process-driven themes that are central to current research on stratigraphy in extensional basins. Until relatively recently, tectono-stratigraphic models for rift basins focused on predictions of basin-scale stratigraphic geometries; these models did not attempt to address the ®ner-scale details of stratal geometry that stratigraphers were documenting in numerous basins. The stratigraphic community, on the other hand, lacked process-driven models to provide a framework within which their detailed observations could be interpreted. For example, it was dif®cult to explain complex temporal and spatial patterns of sedimentary architecture in terms of variations in fault-controlled subsidence. Moreover, few workers had made links between sediment¯uxes as recorded in stratigraphy and the erosional evolution of source areas. Clearly, in such tectonically active basins, footwall erosion and sediment¯ux to the hangingwall basin are linked by the evolution of fault-controlled topography within, and adjacent to, the rift basins. Although a number of books have been published in recent years that have focused on the stratigraphy of rift basins, in particular on the interaction of tectonics and sedimentation (Lambiase, 1995. ; Purser & Bosence, 1998. ; Morley 2000. ) , none of these has dealt speci®cally with the processes governing stratigraphic development in rift basins. This special volume arises out of an upsurge of interest in the mechanisms responsible for the formation of such basins and the processes governing their sedimentary and stratigraphic evolution. In particular, there has been considerable interest in understanding what processes govern subsidence creation, the erosion and deposition of sediment, and stratigraphic preservation in rift basins, and how these processes are interlinked. Much of the impetus for this interest arises out of major advances in our understanding of: N How faults grow and how this in¯uences the development of fault-controlled topography N How erosional catchment systems respond to tectonic and climatic forcing N How sedimentary systems respond to tectonic activity, and climatic and base-level¯uctuations. In the past 10 years, structural geologists and geophysicists have made considerable advances in understanding how tectonic structures grow in space and time through a combination of ®eld studies and numerical modelling. These insights provide the necessary mechanistic frame-Basin Research (2000) 12, 185±194 # 2000 Blackwell Science Ltd 185 work with which basin development and stratigraphic variability can be investigated in terms of the processes of fault growth and fault array evolution. The majority of the papers in this special issue are concerned with the impact of fault growth processes on the development of sedimentary depocentres and stratigraphic patterns in rift basins. These studies clearly demonstrate that a complete understanding of syntectonic stratigraphy can only be attained through application of mechanistic insights into tectonic forcing. A second research strand that has had considerable impact is the study of the geomorphic evolution of tectonically generated topography. A resurgence of interest in constraining and quantifying the erosional processes that wear down fault-controlled topography has been made possible in particular through the development of numerical landscape evolution models and advances in landscape characterization through analysis of digital topographic data (Tucker & Slingerland, 1994. ; Kooi & Beaumont, 1994. ; Densmore et al., 1998. ; Ellis et al., 1999. ) . These models provide, for the ®rst time, the ability to explore quantitatively landscape response to systematic variations in key forcing functions, for example, tectonic uplift rate or precipitation rate. and importantly for basin analysts, as we shall see later, these models provide insights into the temporal and spatial distribution of sediment¯uxes released to basins. Third, much interest has focused on the response of depositional systems in rift basins to external forcing, e.g. faulting, climate change and base-level¯uctuations. Detailed ®eld and subsurface studies have been conducted in numerous rift basins worldwide with the aim of documenting the syntectonic sedimentary architecture, and using this information to extract signals of external forcing (e.g. Gordon & Heller, 1993. ; Dorsey et al., 1997. ; Gawthorpe et al., 1997. ; Peakall, 1998. ; Gupta et al., 1999. , in press. ; Dorsey & Umhoefer, 2000) . . At the same time, numerical stratigraphic models have been widely used to simulate stratigraphic sequences through variation in external forcing (e.g. ter Voorde et al., 1997. ; Hardy & Gawthorpe, 1998. ) . However, few studies have combined ®eld analysis with a numerical simulation of observed stratigraphic geometries. Our motivation for this special issue comes through the realization that only truly interdisciplinary work can lead to further improvements in our understanding of the processes that govern synrift stratigraphy. Our introduction aims to set the scene by brie¯y introducing the contributions to this thematic set in the context of recent, current and future research directions. We hope that by bringing together a series of contributions from a broad range of research areas, we will encourage communication between workers in different ®elds that will lead to a more integrated understanding of extensional basins. It seems clear to us that if we are to achieve this, full collaboration between geophysicists, structural geologists and sedimentary geologists is required, rather than mere'borrowing' of ideas across disciplines. MOTIVATION
doi:10.1046/j.1365-2117.2000.00139.x fatcat:w3q3ewemnzch7ho3ix4iusnwge