Page 112 - Physical Principles of Sedimentary Basin Analysis

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Burial histories

A sedimentary basin consists of strata of different lithologies deposited in different time
intervals. The main data in the burial history are, therefore, the thickness and the lithology
of each layer and the time of the horizons separating the layers. A horizon is taken to be a
surface in the basin of a particular time, and the precise term is therefore chronohorizon.
There is an extensive nomenclature for stratigraphic classification, but here we will not
need more terms than horizon and formation. A formation is here simply the layer between
two consecutive horizons.
A burial history usually has breaks or gaps in the stratigraphical record, either because
of lack of deposition or because of erosion. Such a gap in a sequence of sedimentary rocks
is a hiatus, and an erosion process can partly or completely remove several layers. It is
often difficult to reconstruct what has been eroded; especially, regional erosion processes
make it difficult find places where thickness information is preserved.
The histories of water depth, heat flow and surface temperature complement the burial
history. All these elements go into the modeling of a sedimentary basin, and they apply at
the boundaries of the basin. The development of the basin is modeled by adding layer on
top of layer through the geohistory, eventually with periods of no deposition or periods with
erosion. The deposition history gives the geometry and the material properties of the basin
by processes on the basin surface. The water depth history, surface temperature history and
the heat flow history become boundary conditions for the equations of fluid flow and heat
flow. The geometry of basins is often complicated by additional processes, like faulting
that breaks the geometrical continuity of the strata, or thrust faulting that may even mess
up the chronological order of the strata. These topics are not covered here.
The porosity of the sediments is not constant through time and space. Even similar or
the same lithology often shows a large scatter in the porosity, but lithologies have in com-
mon that the porosity is normally decreasing with time and depth. There are two main
processes that reduce the porosity: mechanical compaction and diagenesis. The uppermost
and unlithified sediments compact mechanically under the increasing weight of sediments
being deposited. When the sediments become buried to the depths of sufficiently high tem-
peratures they are lithified by diagenetic processes that fill in the pore space with cement.
The loss of pore space leads to reduction of the layer thicknesses and compaction. We
assume that diagenetic reactions are local and do not involve “long distance” transport of

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