Exkursion nach Billings

Billings

Am 26.08.2009 ging es los nach Billings.
6 Stunden im Auto sitzen…und wir sind doch erst in Zentral Montana!
Auf den Highways immer gerade aus und ab und an durch eine kleine Stadt und dann wieder gerade aus durch das Nichts, Berge und die Prärie.
Angekommen in Billings hat uns Jörn auf dem Campingplatz begrüßt mit vollem Tatendrang gleich noch loszuziehen um die ersten Sedimentschichten anzuschauen und zu bestimmen.
Billings ist die größte Stadt in Montana, hier ist es noch heißer und trockener!
Jörn hat sich hier die letzten 3 Jahre im Sommer aufgehalten und die Eagle Formation und ihre verschiedenen Sedimentschichen genau studiert. Er hat die Exkursion geleitet und am letzten Tag eine Präsentation vor 30 Geologen aus ganz Montana gehalten.

Wir haben uns 4 Tage dort aufgehalten und alle Schichten genau untersucht.
Den ersten Vormittag haben wir uns mit den Gesteinen aus dem Flussbett des Yellowstone River beschäftigt.

- ultra mafic,
- quarzite,
- felsic,
- intermediate,
- volcanics


Hier ein kleiner Exkursionsbericht der letzten Station in Billings.

The Late Cretaceous Eagle Sandstone outcrops in cliffs exposed around the city of Billings, Montana.  Twenty-four meters of well-exposed sandstone were measured at the top of west-facing cliffs at Phipps Municipal Park.  Sedimentary structures, fossils and ichnofossils, glauconite content, and unit thickness were documented using a tape measure and hand lenses.  A negligible structural tilt (<5°) was ignored.  The measured section was later viewed from below to note broad-scale features.

The basal three meters of the section has glauconitic, marine-burrowed, cm- to dm-interbedded silts and fine sands.  Sands toward the top show some hummocky cross-stratification (HCS).  These silty fine sands are interpreted as a lower-shoreface facies. 

A heavily burrowed sharp contact with woody lag deposits marks the beginning of the next section.  Medium-grain unburrowed low-glauconite trough cross-bedded sands fine slightly upward to meter seven, then transition in to glauconitic interbedded silts and fine sands showing some HCS reaching meter 8.5.  Trough cross-bedding is exposed only on a two-dimensional cliff face but evince a roughly and consistently easterly paleoflow direction.  This section is interpreted as a deepening-upward transition from a fluvial to a marine lower-shoreface setting.

The next section (8.5-17 meters) lies above a sharp incised contact and is a low-glauconite non-burrowed fining-upward medium sand with large (2-meter plus) cross-beds evincing easterly paleoflow direction.  When viewed from below, this prominent cliff-former is incised in to units beneath and contains east-dipping internal features interpreted here to be lateral accretion surfaces within a migrating channel point bar.

The contact with the next slope-forming section is obscured.  Centimeter-scale orange-stained interbedded fine sands, silts, and muds occur from 17 to 19 meters.  Weathered glauconite is inferred from the presence of orange sand grains, but this is uncertain.  No burrows or cross-bedding is observed, though a possible coal or black shale exists.  This grades upward in to burrowed, glauconitic, upward-fining silts and sands.

At 21 meters, a sharp basal contact leads to cm-layered fine to medium glauconitic burrowed sand with some HCS.  These sands continue to the top of the section, though the top 1.5 meters is interbedded with dm-scale beds of cm-layered trough cross-bedded sands showing a southeasterly flow-direction.

Bilder_Billings