Geology topics

Sabine Schmidt

Publications and source records attributed to Sabine Schmidt.

3 recordsLinked to original sources

USGS2026

Subaerially exposed Iceberg Lake sediments: An exceptional record of historical subaqueous earthquake disturbance at the eastern edge of the Alaskan-Aleutian subduction zone

Paleoseismic records are limited in the Yakutat Terrane (eastern edge of the Alaskan-Aleutian subduction zone) due to the extensive ice cover that hinders traditional methods such as trenching of the faults, but lacustrine sediments offer an alternative archive. We investigated lakebed sediments exposed after recent outburst floods (1999 CE) at Iceberg Lake, a glacier-dammed basin whose stratigraphy was revealed after the lake drained and partially eroded. We logged outcrops across the basin and sampled their sediments. Between annually laminated background deposits, we identified sediment gravity flow beds and in-situ soft-sediment deformation structures (convolutions, sand blows, and fractures) interpreted to be earthquake-induced. Our age model links some of the uppermost depositional and deformational events to the 1958, 1964 and 1979 CE earthquakes. These results demonstrate that Iceberg Lake was a sensitive recorder of seismic shaking and its sediments hold strong potential for producing a paleoseismic record for the northern Yakutat Terrane.

Alaska↗

USGS2017

Varve formation during the past three centuries in three large proglacial lakes in south-central Alaska

The sediments stored in the large, deep proglacial lakes of south-central Alaska are largely unstudied. We analyzed sediments in 20 cores, up to 160 cm long, from Eklutna, Kenai, and Skilak Lakes, using a combination of repeated lamination counting, radionuclide dating, event stratigraphy, and tephrochronology. We show that the characteristically rhythmic layers were deposited annually. Most of these glacial varves consist of one coarse-grained base and a fine-grained top, but varves composed of multiple coarse-grained turbidite pulses are common too. They are likely related to successive episodes of high sediment discharge during flooding, and they become more frequent in all three lakes, along with increased sedimentation rates, during the nineteenth century late phase of the Little Ice Age. These flood turbidites were generated by rain events and intense melting of snow and ice. Other (mega) turbidites are a result of earthquake-triggered slope collapses (e.g., A.D. 1964). Some event layers are present in all three lakes. In addition, the annual time series of varve thickness (normalized annual sedimentation rate) are significantly correlated among the three lakes (ρ > 0.27; p < 0.001). Differences between the varve thickness records can be attributed partly to the dam construction at Eklutna Lake and outbursts from an ice-dammed lake at Skilak Lake. Geomorphologic differences among the catchments result in further differences in sedimentation patterns in the three lakes.

Alaska↗

USGS2017

Paleoseismic potential of sublacustrine landslide records in a high-seismicity setting (south-central Alaska)

Sublacustrine landslide stratigraphy is considered useful for quantitative paleoseismology in low-seismicity settings. However, as the recharging of underwater slopes with sediments is one of the factors that governs the recurrence of slope failures, it is not clear if landslide deposits can provide continuous paleoseismic records in settings of frequent strong shaking. To test this, we selected three lakes in south-central Alaska that experienced a strong historical megathrust earthquake (the 1964 M w 9.2 Great Alaska Earthquake) and exhibit high sedimentation rates in their main basins (0.2 cm yr − 1 –1.0 cm yr − 1 ). We present high-resolution reflection seismic data (3.5 kHz) and radionuclide data from sediment cores in order to investigate factors that control the establishment of a reliable landslide record. Seismic stratigraphy analysis reveals the presence of several landslide deposits in the lacustrine sedimentary infill. Most of these landslide deposits can be attributed to specific landslide events, as multiple landslide deposits sourced from different lacustrine slopes occur on a single stratigraphic horizon. We identify numerous events in the lakes: Eklutna Lake proximal basin (14 events), Eklutna Lake distal basin (8 events), Skilak Lake (7 events) and Kenai Lake (7 events). The most recent event in each basin corresponds to the historic 1964 megathrust earthquake. All events are characterized by multiple landslide deposits, which hints at a regional trigger mechanism, such as an earthquake (the synchronicity criterion). This means that the landslide record in each basin represents a record of past seismic events. Based on extrapolation of sedimentation rates derived from radionuclide dating, we roughly estimate a mean recurrence interval in the Eklutna Lake proximal basin, Eklutna Lake distal basin, Skilak Lake and Kenai Lake, at ~ 250 yrs., ~ 450 yrs., ~ 900 yrs. and ~ 450 yrs., respectively. This distinct difference in recording can be explained by variations in preconditioning factors like slope angle, slope recharging (sedimentation rate) and the sediment source area: faster slope recharging and a predominance of delta and alluvial fan failures, increase the sensitivity and lower the intensity threshold for slope instability. Also, the seismotectonic setting of the lakes has to be taken into account. This study demonstrates that sublacustrine landslides in several Alaskan lakes can be used as reliable recorders of strong earthquake shaking, when a multi-lake approach is used, and can enhance the temporal and spatial resolution of the paleoseismic record of south-central Alaska.

Alaska↗