Jökull


Jökull - 01.01.2016, Page 63

Jökull - 01.01.2016, Page 63
The marginal zone of Tungnaárjökull, Iceland At a depth of approximately 3.2 m from the top of the profile, lithofacies A2 contains a layer of ripple cross- laminated medium- and fine-grained sand. Immedi- ately overlying the sand is a several-cm-thick layer of horizontally laminated silt which contains a large (7-cm diameter) cobble which distorts the lamination (Figure 9B). Overlying the silts is a 2 cm thick layer of massive sand and fine gravel (SGm). Sediments of unit A2 are interpreted as having been deposited in standing or low-flow water condi- tions. Their character, however, indicates a higher en- ergy sedimentary environment than the underlying A1 lithofacies. The estimated deposition velocities of the currents responsible for the deposition of lithofacies A2 sediments range from 7.6 to 12.8 cm/s (Koster, 1978). An increase in the grain diameter of the sand fraction in the top of the unit indicates an increase in flow energy. The domination of sand, as well as the presence of ripple cross-lamination, indicates trans- portation of the material in bottom and fractional sus- pension. The layer of ripple cross-laminated medium- and fine-grained sand (Sr) is interpreted as episodes of lower flow regime conditions. The layer of silt (Fhd), which contains a cobble of approximately 7 cm di- ameter, interpreted as a dropstone (Figure 9B), was deposited in an environment of standing water. A thin layer of SGm lithofacies proves the episodic growth of current energy with the participation of traction. Unit A3 is 2.05 m thick (Figure 8A) and comprises horizontally stratified gravel, sandy-gravel, and sand (Gh, SGh, Sh). The base of the unit, between 2.0 to 1.5 m depth, is marked by 0.5 m thick gravel, which is in turn overlain by a sand dominated sequence (fine to coarse sand) containing occasional sandy-gravel beds. The MPS in the gravel and sandy-gravel layers ranges from 80 to 150 mm. Unit A3 was accumulated under the conditions of an upper flow regime and high-energy current. Under these conditions, sand and gravel grains are thought to have been transported in traction and bottom sus- pension (saltation). The average water flow (deposi- tion velocities) rate, estimated on the basis of average grain diameter, is from 30 to 40 cm/s (Koster, 1978). To determine the energy of the depositional environ- ment, the flow competence indicator (MPS) was also used. This was calculated on the basis of the ninety- fifth percentile (D95) and field measurements of the average axis (b) of the ten largest grains from the monogenetic facies Gh, GSh and SGh. The current velocity values calculated from D95 range from 0.7 to 0.8 m/s, while those based on field measurements range from 1.1 to 1.4 m/s. The lowest unit of profile B, (B1) has a thickness of 1.2 m on top of dead glacial ice (Figure 8B). Unit B1 consists of sandy-silty lithofacies of massive struc- ture and wavy laminating or climbing ripples (Figure 9C). The sediments of unit B1 were deposited in an environment of standing water (Fm, SFm, FSw, SFw, Sw) or under low-energy flow conditions (Src, SFrc). Unit B2 has a thickness of 1.75 m and con- sists mainly of fine-medium-coarse grained sand. The lower 1.4 m of the unit has a well-developed low- angle cross stratification (layers dip 10◦, with dip direction of approximately 100◦) which records a palaeoflow direction towards the east (Figure 9D). The upper part of lithofacies B2 comprises a 25 cm thick layer of horizontally bedded sand (Sh) capped by a 10 cm thick bed of massive silt (Fm). The upper part of unit B2 defines a coarsening upward cycle. The thick series of sediment with low-angle cross stratification (Sl) and horizontal stratification (Sh) at the top, is interpreted as being deposits of an alluvial fan. The massive silt which caps Unit B2 can be inter- preted as having been deposited in an environment of standing water in the channel. Unit B3 (3.35 m thick), consists mainly of grav- elly sand inclined layers of deposits with boulders of massive structure and clast-supported (Gcm, GSm SGm), secondary horizontal or low-angle cross strat- ification (GSh, Sl, SFl). At the top of this unit’s sedi- ments there is a fining upward cycle. The maximum diameter of the grains in the gravel–boulder layers range from 100 to 140 mm. The gravel-rich GSh lithofacies was accumulated under upper flow regime and high-energy current con- ditions. The mainly inclined layers of the sediment of unit B3 (Figure 8B) were probably the result of a de- bris flow (e.g. Zieliński, 2014), attested by the mas- sive structure of the deposits, formed as several thick series of boulder–gravel–sand diamicton, and the low JÖKULL No. 66, 2016 63
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