Jökull - 01.12.1990, Side 6

tions. These types have quite different paleoclimatic
implications.
During the last decades there has been an empha-
sis on the application of facies analysis to the study of
glacial deposits (Eyles et ai, 1983; Eyles and Miall,
1984; Shaw, 1987). These studies have resulted in the
detection of a variety of depositional processes within
the glacial environment. Debris transported by the ice
is released during its wastage and deposited directly
or carried away by some agent before deposition. The
simplest case involves the formation of melt-out till
when the debris is released in its original position in the
ice. It may melt out from a moving ice or after its stag-
nation, both from the base and the surface. Lodgement
till is formed when pressure increases during move-
ment of the ice. A release of debris from the base
of the ice occurs when the pressure-melting point is
reached. The most complex case occurs where sheets
of ice are moved over each other along shear-planes or
by compressive flow, some release of debris may take
place up on the ice surface at the front of these planes.
Debris released from the ice may also be carried away
by some agent before deposition, such as by slumping
and mass flow processes.
Simultaneous with the development of new meth-
ods for differentiation among glacial facies, a plethora
of genetic models for volcaniclastic and mass-flow
deposit have also been developed (i.e. Fisher and
Schmincke, 1984; Lajoie, 1984; Cas and Wright,
1988). In both systems, the emphasis is placed on
facies analysis and multiple sedimentological criteria,
largely because many of the sedimentary characteris-
tics that form during glacial deposition, can also form
during mass-flow processes.
Although hyaloclastite varieties cover up to 20-
30 % of Iceland, very little studies have been made
to explain the origin and mode of emplacement of the
various types of hyaloclastites. However, the general
theory is that they were formed either by explosive
fragmentation and or granulation of basaltic lavas due
to rapid chilling in contact with water or ice. The most
common morphologic products of subglacial volcan-
ism, prevalent in the late Pleistocene to Recent period
in Iceland, are table mountains and hyaloclastite ridges
(Pjetursson, 1900; Nygaard, 1940; Kjartansson, 1943,
1966a, 1966b; Einarsson, 1960; Peacock, 1962; van
Bemmelen and Rutten, 1955; Einarsson, 1966; Jones,
1970). These landforms are thought to have origi-
nated by the build up of fragmental material above
a subglacial central vent or a fissure. Hyaloclastite
sheets which cover large areas instead of producing
narrow emerged ridges are thought to form as a result
of brecciation of lava during flowage in water or be-
neath ice (Einarsson, 1946, 1962; Walker and Blake,
1966; Sæmundsson and Jóhannesson, 1980; Bergh,
1985).
However, none of the above mentioned deposi-
tional mechanisms for hyaloclastites is able to infer
the nature and extent of glaciations. Strictly speaking
only the subglacially accumulated material released
by melting or pressure melting of ice stands as a
direct evidence of the extent of glaciation (Lawson,
1979; Lundqvist, 1988). In order to conclude about
the paleoclimatic implication of basal tills, a detailed
mapping, correlation and dating of glacial deposition
over a large area is essential.
The first detailed and systematic facies analysis
made on Icelandic sediments within a stratigraphic
succession was carried out on the Tjömes strata in
northem Iceland (Eiríksson, 1979, 1981, 1985). A
variety of deposits were recognized which represent
a rhythmic character within a vertical column of the
thick Quaternary strata on Tjömes. A model for sed-
imentation during glacial-interglacial cycles in a vol-
canic environment was presented distinguishing be-
tween 14 separate glaciations, the oldest dated at 2 Ma
(Eiríksson, 1979,1981,1985; Albertsson, 1978).
The purpose of this paper is to draw attention to the
importance of facies analysis of diamictites (unsorted
sedimentary rock) in environmental interpretation of
ancient rock successions. It outlines the results from
a detailed sedimentologic work made on several di-
amictite units which are found interbedded between
lava flows of the stratigraphic succession of the upper
Borgarfjörður and the Hvalfjörður area. A detailed
description of each sedimentary facies found within
these areas and methodology used to differentiate di-
amictites will be the subject of a forthcoming paper
(Geirsdóttir, in prep.). The results imply more varied
depositional processes for the origin of the diamic-
4 JÖKULL, No. 40, 1990