Page 45 - Petroleum Geology

INTRODUCTION

A significant geological feature of sedimentary basins revealed first by coal
mining, then much more widely by drilling for petroleum, is the common oc-
currence of faults that have a thicker sedimentary sequence in the down-
thrown block than the correlative sequence in the upthrown block. Such faults
have been interpreted from the earliest days of their recognition as faults
that moved while the sediment was accumulating. They are not only evidence
of deformation of sedimentary basins early in their development, but also
interesting for the light they throw on the processes of sediment accumula-
tion. They are an integral part of sedimentary basin development.
There is similar evidence of contemporaneous folding of strata into anti-
clines in which the rock units are thinnest over the crests. These are but varia-
tions on the theme discussed in the first chapter, that subsidence determines
the thickness of rock units.
The reason why such faults and folds are rarely identified at the surface
by surface geologists is that their recognition depends on consistent, but re-
latively slight changes in thickness. Such changes are readily detected in coal
mines and boreholes - and, since about 1970, in seismic record sections -
but rarely detectable at the surface because thicknesses measured at the sur-
face are composite over an area.
Considerable misunderstanding of these structures has resulted from the
application of the old axiom “Folds and faults are younger than the rocks
folded or faulted”, because this has usually been interpreted on a time scale
that is much too long (e.g., Miocene rocks are faulted, therefore the faulting
is post-Miocene). The axiom is better rephrased “Folds and faults are not
older than the rocks folded or faulted” to allow consideration of contempo-
raneity .

DEFINITIONS AND TERMINOLOGY

Growth structures are structures in which the variations in sedimentary
thickness show a close relationship to the structure itself. They are interpreted
as being the result of deformation that was contemporaneous with sediment
accumulation. Growth structures are normally thought of as faults or anti-
clines, but they may also be monoclines and synclines. They are local variations
in the development of a sedimentary basin, but the sedimentary basin itself
is a growth structure in the broadest sense.
It is difficult to define growth structures without including the interpreta-
tion of them in the definition. Drape, or supratenuous folds are not growth
structures in spite of the fact that the sedimentary thicknesses show a close
relationship to the underlying structure. Likewise, contemporaneous deforma-
tion, as used by Billings (1954) and subsequent writers, refers more to the