Jökull - 01.01.2013, Side 48

J. Helgason and R. Duncan
Table 4. Main characteristics of four erosional stages in Svínafell’s stratigraphy. – Helstu einkenni fjögurra
rofskeiða í jarðlagastafla Svínafells.
Stage Erosive Degree of Environm. Main lithology on Unconformity Estim. depth Estim. age (Ma)
agent volcanism erosive surfaces erosion (m)
1st river continuous subaerial sediments, minor or < 25 pre-ice age
erosion e.g., pebble absent ∼ 4 to ∼ 2.7
conglom.
2nd dominant no volcanism mainly minor tillite clear > 250? Early Quat.
glacial observed subglacial or conglom. ∼ 2.7–1.95
erosion
3rd dominant subglacial subaerial extensive erosional > 100 Upper-Mat.
glacial and and sedimentary unconform. ∼ 1.95–0.781
erosion subaerial subglacial deposition
4th dominant extensive subglacial extensive major >1000 Brunhes
glacial subaerial and accumul. valley chron
erosion volcanism subglacial of volcanics deepening <0.781
Stage 1. As strata in Svínafell that predate the Matuyama chron (> ca. 2.6 Ma) consist entirely of lavas we assume that
erosion was minor (< 25 m) during the Tertiary and that rivers were the dominant erosive agent.
Stage 2. When glacials became frequent, roughly after 2.6 Ma, glaciers began to form and shape the earliest valleys. Into
one such lavas of Olduvai age were deposited by Svínafell. When stratigraphic position lavas of this age in Hafrafell and
Svínafell is compared we conclude that valleys at this time were at least 250 m deep.
Stage 3. During Upper-Matuyama or from about the Olduvai chron to the onset of Brunhes (about 1.945 to 0.781 Ma) lavas
were carved down by over 100 m.
Stage 4. Volcanism from the Öræfajökull volcanic center during Brunhes was extensive by Svínafell as well as erosion and
valleys continued to deepen by at least 1000 m.
ite basalt lava formations SV3 and SV5, respectively.
A clear unconformity is seen in relation to this sur-
face, i.e., 14◦/75◦, and lava flow dip is notably greater
above it. Formation SV4 has two units, i.e., an upper
unit of sandstone and a lower unit of pebble conglom-
erate. The conglomerate is 3 m thick with boulders up
to 1 m in diameter. The conglomerate lacks bedding,
has poor sorting and pebbles are angular. Matrix is
heterogeneous with grain size mostly less than 1 cm,
gray to brown.
SR2. Erosional development through the formation of
surfaces SR2 to SR4 is shown on Figure 13a. For-
mation SV6, deposited during erosion stage SR2, con-
sists of a 3 m thick sandstone and pebble conglom-
erate. This conglomerate is intercalated between the
lower basalt lavas (formations SV1 to SV5) and the
Skjólgil lavas (formations SV7 to SV9). The Skjól-
gil N-lavas are correlated with the Olduvai polarity
chron (1.78–1.95 Ma) based on lithologic and mag-
netic similarity to N-lavas in Hafrafell west of Svína-
fellsjökull, i.e., formation HF29 at about 560–580 m
elevation. It follows that surface SR2 is older than ap-
proximately 1.95 Ma. Despite only minor sediment
deposited on erosion surface SR2 we argue that a ma-
jor erosional unconformity coincides with this sur-
face. Thus, in Hafrafell, reversely magnetized lavas
of lower Matuyama age have a thickness of at least
350 m (Helgason, 2007). There, a second sequence
dated and correlated with Olduvai, is found as val-
ley filling lava flows, banking up against the 350 m
thick R-lava flow sequence of lower Matuyama age.
The abrupt reduction in thickness of R-lavas of lower
Matuyama age in Svínafell, over a distance of only 2
km, is interpreted to result from relief amounting to at
least 230 m. This erosion presumably took place dur-
ing lower Matuyama time, approximately 1.95 to 2.58
48 JÖKULL No. 63, 2013