File 1450/1919 ‘Mesopotamia & Kurdistan: Geological Reports on’ [‎104r] (222/522)

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Final Summary Report.
INTRODUCTION.
In the series of reports written during the process of this survey, I have put forward freely
Opinions and hypotheses as they gradually ‘developed with the work. It is now proposed to review
briefly some of the points and to summarize the extent to which these opinions and suggestions have
been confirmed or modified. No radical change of opinion has been found necessary, but the
* scheme of classification has naturally expanded and the theory of unconformity tested.
UNCONFORMITY.
In Report No. 2 it was stated that, in view of the marine character of the Jabal Hamrin Pars
series and the fluviatile nature of the succeeding Red Clays and Sandstones, and also in view of the
correspondence in character of these Pars beds with Dr. Pilgrim’s gypsum-bearing Lower Pars on the
one hand, and the Red Clays and Sandstones with his Bakhtiari series on the other, the boundary
between the two series in the Jabal Hamrin might be an unconformity, and possibly one of some
magnitude. It was also stated that an extended examination of this boundary in other areas would
be necessary to prove this one way or the other, and that there was no evidence at all in the portion
of the Jabal Hamrin investigated, to prove that this unconformity was anything more than the break
consequent on the change from a marine gulf to a river. It has been found possible to extend the
last statement to all areas subsequently surveyed. The top bed of the Pars varied in nature from a
fossiliferous limestone to a gypsum band, but to what extent this was due to lateral variation or to
unconformity, it would be difficult to say ; the impression gained was that this break involved very-
little erosion between the two periods of deposition. Some geologists would perhaps not consider
this large enough to be spoken of as an unconformity, and Mr. J. Brown has gone so far as to class
the top gypsum or limestone band and the beds of red clay underneath it as ‘ Passage Beds ” (or
Middle Pars), by which the gypsiferous series passes up into the overlying red clays and sandstones.
It is purely a matter of terminology, and I see no objection to the use of either of the terms
“ unconformity ” or “ passage beds ”, provided it is made plain what is meant by the term. My
interpretation of the sequence of events is as follows The gypsum-bearing series was deposited in
a gulf of no great depth, which became silted up and gave place to a river, with perhaps an inter
mediate phase of salt lagoons. To effect this change land must have appeared and erosion to some
extent at least have taken place. In this sense, therefore, there must be unconformity everywhere,
while here and there erosion might be very appreciable. If it were everywhere an unconformity
of any size, we must presuppose a widespread upheaval of a very gentle epeirogenic kind, but I am
more inclined to the view that corrugation of the beds commenced early in the Tertiary period ; if
so the fairly persistent nature of the upper portion of the gypsum-bearing series and the constant
way in which it is, with few exceptions, followed by the same zone of red clays (zone “a” of the •
Red Clay and Sandstone Series), are more in favour of the view that there was a minimum of erosion.
The few exceptions in the composition of zone “a ” were all obviously due entirely to local lateral
variation. Before the final establishment of lagoon or fluviatile conditions, there were premature
occurrences of such conditions, followed by temporary submergence beneath the gulf waters. The
last premature change of this kind occurred very generally over wide areas, producing the belt of
red clays just below the topmost sedimentary gypsum band or fossiliferous limestone. This belt
including the topmost marine bed I take to be Mr. Brown’s “ Passage Beds ”. It denotes a final-
oscillation between marine and lagoon or fluviatile conditions ; on my maps it has been included
with the gypsum-bearing series below (" Pars”, or “ Lower Pars” of Reports 1—9). I am still of
opinion that localities will be found in which erosion has been greater, and the unconformity visible
or demonstrable ; it may very possibly be demonstrable in some of the areas mapped when more
adequate maps are available.
CLASSIFICATION.
In view of the doubtful size of the unconformity the following question arises “ If the
gypsum-bearing beds are to be assigned to the Lower Pars, how^ can the succeeding Red Clays and
Sandstones be classed as Bakhtiari?” As stated in Report No. 2, the “Gypsiferous Group” of
Loftus was split up by Dr. Pilgrim into a lower series which he called Pars and an upper which be
called Bakhtiari, chiefly, I take it, because the older beds are marine and the younger fluviatile, and
because of a dividing unconformity. Now since the gypsum-bearing strata of the area I have been
surveying are certainly marine (excepting the small preliminary lagoon or river phases mentioned
above), and the Red Clays and Sandstones certainly lagoon or fluviatile or probably both, and since
the boundary between is the only place where a widespread unconformity can possibly be allowed
to exist, it seems to me very probable that taxially the Red Clays and Sandstones are the equivalent
of Pilgrim’s “ Bakhtiaris”, and, if the erosion of the gypsum beds along the boundary be small,
these beds must be correlated with the higher horizons of the Pars and not with the lower. For
the above reasons I should noW ( prefer to avoid the term “Lower Pars” used in my first nine
reports and to substitute simply “ Pars,” until the question is cleared up by connected mapping
between the Mosul-Baghdad-Kirkuk area and the Persian Gulf The historical term used to describe the body of water between the Arabian Peninsula and Iran. . Dr. Pilgrim concluded that the ;
unconformity, in the Persian Gulf The historical term used to describe the body of water between the Arabian Peninsula and Iran. area, even where his upper Pars were present, was of considerable
size and the result of gentle undisturbed upheaval.
Pilgrim’s basal gypsum stage is described as passing up into his Ostr^a verleti stage in which
mterbedded gypsum bands occur. He says, “ I am not prepared to deny that the lowest beds of the
Ostraa verleti division occurring immediately above the typical development of the gypsum beds ue
not contemporaneous with portions of the Pars series further north which I have classified with the