Page 10 - i1052-5173-30-2

Incision history may be affected by ongoing uplift, but this is this idea, cliff and tributary river profiles along the <17 Ma Grand
debated. Absent strong spatial gradients in rock strength or rock Wash Cliffs are considerably less steep than those in western
uplift rate relative to base level, rivers evolve toward smoothly Grand Canyon despite similar geology and climate, implying that
concave-up profiles. In contrast, major slope-break knickpoints or the final ~1 km of relief generation in westernmost Grand Canyon
convexities in the river profile existing on the Colorado River at is <10 Ma (AD).
Lees Ferry and on the Little Colorado River near Cameron give Regional and global analogs were also discussed. Mantle-
rise to “double concave” river profiles. These knickpoints coincide driven uplift at a rising plume head can be modeled to leave pre-
with the top of the Kaibab Limestone surface, suggesting a con- dictable stratigraphic patterns in the sedimentary record of many
trolling influence of rock strength. These and other profile con- continents (AF). The Rio Grande, on the other side of the continen-
vexities are at least partly controlled by rock strength (KW, KC) tal divide, extended its length in a downward direction about six
and may be “hung up” in harder rocks, complicating interpretation million years ago at the same time Grand Canyon was becoming
(AD, KC). Projection of the restored level of the pre–6 Ma Little integrated, which may implicate climatic changes near the end of
Colorado paleoriver profile through the proposed East Kaibab the Miocene (MR). River incision and profile analysis in the central
paleocanyon suggests that the Esplanade bedrock bench of west- Anatolian region of the eastern Mediterranean supports very young
ern Grand Canyon could have been cut at this time (KW), tectonic uplift caused by slab breakoff (TS). Himalayan and
although a new date of 3.3 Ma of basalt on Whitmore Hill that Andean rivers show double-concave profiles with major knick-
rests on Hermit Shale shows that the entire Esplanade surface had zones, a delay between uplift and incision in knickzones, fault-
not yet been exposed at this time (KK). control on knickzones, and tectonic rather than climate controls
Landscape evolution of the region has been influenced by pro- on incision (PvdB). Along the Nile River, the change in base level
nounced differences in rock erodibility (rock strength) that give during the Messinian drawdown, and the uplifting Ethiopian
rise to the characteristic cliff and bench morphology of canyon Plateau headwaters provide possible direct comparisons to
walls and the Grand Staircase. Erosion rates can be quantified by Colorado Plateau evolution with similar multi-stage uplift and
measuring the concentration of cosmogenic Be in sands depos- potential ongoing mantle-driven dynamic uplift (CF).
10
ited by river tributaries (AH). These data show significant scatter Geoscience outreach is needed to improve global science
but with averages in eastern and central Grand Canyon generally literacy. Challenges include language barriers, validating tested
similar to independently measured incision rates (KW). In western from pseudoscience, citing and crediting sources, and outreach
Grand Canyon, tributary profiles are suggestive of sustained to developing countries. The “Learning Geology” Facebook page
quasi-steady river incision since integration (AD). However, it is (headed by MQM) reaches 137,000 geoscience learners interna-
also possible that incision on these tributaries has ceased, but their tionally and provides an ongoing successful example of geoscience
form is preserved by an armoring of large boulders that inhibits outreach through social media. Informal science education at
further incision and topographic relaxation, potentially consistent Grand Canyon in partnership with Grand Canyon National Park
with a longer history of western Grand Canyon (MD). Contrary to offers continued opportunities.

10 GSA Today | February 2020