Vestmarka Formation

The Vestmarka Formation takes its name from Vestmarka, a forested area west of Oslo. The Vestmarka Formation consists of the oldest preserved lavas in the Oslo Rift and forms the lower part of the Krokskogen Group. The Krokskogen Group consists of lava flows and sedimentary rocks from the Late Carboniferous and Permian.

Larsen (1978) defined the Vestmarka Formation as the rocks from the Kolsås basalt member (B1) up to and including rhombic porphyry lava RP5. In thickness, the formation consists mainly of lava flows, but it also includes sedimentary rocks between the lava flows. The Vestmarka Formation is separated from the Kjaglia Formation, the other formation in the Krokskogen Group, due to differences in distribution, thickness and type of lava flows in the two formations. RP5, the last lava flow in the Vestmarka Formation, is a thick lava flow that represents a very active volcanic period in the rift formation. While the overlying RP6 is missing in several places, presumably due to a period of faulting. In the Vestmarka Formation, basalts with the exception of the Kolsås basalt member (B1) are rare, whereas basalts are more common in the overlying Kjaglia Formation (Larsen, 1978).

In Larsen (1978), 9 lava flows are known: Kolsås basalt member (B1), Rhombe porphyry RP1 (Kolsås type), Rhombe porphyry RR2a, Rhombe porphyry RP2b, Rhombe porphyry RP3a, Rhombe porphyry RP3x, Rhombe porphyry RP3b, Rhombe porphyry RP4 and Rhombe porphyry RP5. The Rhombe porphyries were originally divided according to the size and shape of the phenocrysts. As it was reasonable to assume that similar phenocrysts came from the same source, and thus from the same lava flow or from closely following lava flows. Other methods for distinguishing lava flows are to find sedimentary layers, which were built up between the lava flows, or to find a brecciated zone with the top of one lava flow and the bottom of the next. However, that requires good exposures at a location that shows exactly that transition, or drill cores that show the transition. Recent drill cores, taken in preparation for a new tunnel for the Ringeriksbanen railway, show that RP1-RP4 consists of at least 38 separate lava flows (Svensen et al., 2024). So, the old numbering system should be thought of as lava-series of consecutive lava flows rather than a single flow, although some may be single flows.

Kolsås basalt member (B1)

Kolsås basalt (B1), at Kolsås. Close-up of the blueish-gray basalt at Kolsåstoppen, which can be seen as a clearly visible layer in the cliff face of Kolsåstoppen, see the picture of the southern end of Kolsåstoppen above. At the top of the layer, today the border with RP1, which formed closer to the surface the basalt is vesicular. As the pressure at the top was lower, the gas in the lava formed bubbles, which have solidified as cavities in the rock (vesicles). The picture of it here.

Quartz tholeiitic basalt

The Kolsås basalt has been mapped by NGU as a quartz tholeiitic basalt. That is, a subalkaline, hypersthene-normative basalt (on average, the chemistry of the rock corresponds to the mineral hypersthene ((𝑀𝑔,𝑒)𝑆𝑖𝑂3)) with orthopyroxene and/or pigeonite in addition to clinopyroxene and Ca-rich plagioclase (Sigmond et al., 2021).

The Kolsås basalt is mainly aphyric and grey to a blueish-grey in colour. In 1945 Sæter introduced the abbreviation B1 which stands for the first (oldest) basalt in the stratigraphy of the Oslo Rift, and B2, B3 … etc. for younger basalts Oftedahl (1960). Stratigraphically The Kolsås basalt belongs to B1, and is the oldest lava outcrop in the Oslo area. The Kolsås basalt consists of a single lava flow (Dons & Györy, 1967; Weigand, 1975), but B1 further south appears to come from a different source based on its chemistry (Weigand, 1975).

Sedimentary rocks between B1 and RP1

Quartz sandstone

In many locations, a sandstone layer is found between the Kolsås basalt and RP1. This sandstone is often interpreted as eolian sand dunes, but in some places it may be water deposits (Dons & Györy, 1967). At Grefsenåsen this layer is a 1-2 m thick, light-colored, fine-grained quartz sandstone (Holtedahl, 1957). At Kolsåstoppen this layer is locally missing at the most common path up to the summit, but is present on the western side of Kolsåstoppen. The sandstone is up to 1 m thick in depressions in the underlying basalt. The sediments in this layer at Kolsås are varied, the sandstone occurs in several colors: red, brown, white, greenish or dark gray. It also contains larger fragments of vesicular basalt, quartz and some feldspar fragments that are probably from rhombic porphyry (Dons & Györy, 1967). At Semsvannet there are information plaques that tell you that this layer is red desert sand that blew in between the depositions of B1 and RP1. Today this layer is an up to 1 m thick charkoal-colored slag/sandstone layer.

The rhomb porphyries RP1 to RP5

Top left: Rhombe porphyry RP1 (Kolsås type), as it is at Kolsåstoppen (59°55′3.60148″ N, 10°31′5.41694″ E).

Top right: Rhombe porphyry RP2a, as it is at the north-east side of Furuåsen (59°52′32.22732″ N, 10°25′59.56566″ E) in Asker, north of Semsvannet just near Stokkermyra.

Bottom left: Rhombe porphyry RP2b, as it is at the south side of Furuåsen in Asker, north of Semsvannet just near Stokkermyra.

Bottom right: Rhombe porphyry RP3a, as it is at the top of Furuåsen in Asker, north of Semsvannet just near Stokkermyra.

Sedimentære lag mellom RP-lavaene

Between the rhombic porphyries there are also sedimentary layers. There is little literature on the sedimentary layers between the RP lavas in the Vestmarka Formation, at least there is little that is easily accessible. This is possible because these layers are more local, as they are not deposited in a large ocean area like the older sedimentary rocks in the Oslo area. Instead, it seems that these are deposits in rivers, river fans, small lakes and wind-blown sandstones in a desert climate (Larsen et al., 2008).

Of the layers mentioned in the literature are the sedimentary layers between RP2 and RP3 and between the different RP3 series at Retthelltjern, up from Sundvollen at Tyrifjorden. Larsen (1978) describes conglomerates of rhombic porphyry that vary from well-rounded and sorted conglomerates deposited in water, to poorly sorted conglomerates with irregular fragments that are possibly a broken lava flow filled with wind-blown sand. Other conglomerates in the Vestmarka Formation probably also fall within this spectrum.

The conglomerate between RP2a and RP2b north of Furåsen in Asker (approx. (59°52′32.91775″ N, 10°26′6.70917″ E)). It looks like the conglomerate is mainly a rhombic porphyry conglomerate with rocks from RP2a, but where I looked at the rock it was just a pebble conglomerate, so it was hard to tell if there are any other rocks included.