Reports
&
Analyses

Technical notes on the Braid-scarp

This page describes a feature that may have been generated by faulting near Lake Iliamna, and provides greater depth to what is presented on our possible Lake Clark Fault page.

The 'Braid-scarp' is a step in elevation of about 1 m that runs across a Late Pleistocene or early Holocene braided outwash plain. It is broken into two segments, each over a kilometer long, with a gap of about 1 km in the middle across a series of kettle-lakes. The plain has been 'cryoturbated' or disrupted by ice in the soil, but on the Braid-scarp itself this disruption is much less. It is visible in aerial photos because the tundra that grows on most of the plain is sparse and gravelly along its length.

Methods

We dug two trenches across the steepest portion of the Braid-scarp, one near the northeastern end of the feature, and one southwest of the gap with kettles. We chose sites that did not appear cryoturbated, and where the scarp was relatively constant along its length. Each trench was about 4m long, a meter wide, and varied between 1 and 2 m depth.

The stratigraphy in the trench wall was primarily fluvial cobbly sand and gravel capped by silt. Often layers were indistinct and variable in character and thickness. The largest cobbles were 20 cm or more across, and often broke otherwise continuous contacts.

To survey the trench wall, we set a level line and measured along that line and down from it to features that we were sketching. Sketches were done on graph paper at a scale of 1 inch to 20 centimeters or about 1:8.

Our resulting sketches are shown in our Braid-scarp trenches figure. Generally the stratigraphy was reasonably clear near the ends of the trenches, but directly below the steepest slope of the Braid-scarp we were unable to follow any layer all the way from one side to the other.

Fluvial Scarp Interpretation

If the Braid-scarp is a diffused fluvial scarp, then this region of unclear sedimentary structure is likely to be the collapsed wall of the scarp, where layers were mixed after the river retreated and the erosional bank collapsed to vertical. At the southwestern pit this interpretation fits reasonably well, however in the northeastern trench we couldn't identify any configuration of an erosional scarp that could explain the observed stratigraphy. There were no layers that seemed to correspond to sloping beds formed early in the diffusion of an erosional scarp, and the un-bedded section was sloped up to the south, opposite of what would be expected for a fluvial erosional scarp.

Finally, with the fluvial scarp interpretation it is surprising that none of the main surfaces of the scarp are apparent: the lower surface, the upper surface, or the sloping surface between. The upper surface may simply have been cryoturbated too extensively to be seen, but the other two surfaces should have been quickly buried by collapse and diffusion of the upper part of the scarp and protected from cryoturbation.

Active Fault Interpretation

In contrast, interpreting the un-bedded sections in both pits as disrupted layering resulting from shear on a fault fit our observations nicely. It also explained an odd sub-vertical feature near the edge of the southwest trench, a likely crack.

A preserved ground surface may be preserved in the southwest pit. In this pit, the sharpest contact is silt sharply overlain by sand. This surface slopes down steeply from the disrupted area. Since the silt, likely aeolian loess, is absent deeper in both pits, it may be that it was a layer that formed on the surface like the modern surficial silt, but it was buried by sand washed down from coarser sediment exposed by faulting. This would explain the unusual sharp sand-over silt contact. If this is a preserved ground surface, the feature at the southeast end of this pit is actually earthquake induced cracking, and this provides evidence for at least two ruptures here.