Quaternary lacustrine braid deltas on Lake General Carrera in southern Chile

Abstract

ABSTRACT.A series of Quaternary lacustrine braid deltas on the shores of Lake General Carrera in southern Chile formed as a result of falls in lake level. Each delta comprises two components, the delta fan and the beach embayment, each in turn comprising an onshore subaerial and an offshore subaqueous element. The delta fans have a classic Gilbert form with a braided delta top and a steeply inclined delta front. Adjacent to the fans are concave beach embayments that formed parallel with the deltas as they prograded into the lake. The delta sediments consist predominantly of matrix-and clast-supported gravels to a total composite thickness exceeding 400 m. The depositional environments are dominated by ephemeral river fl ooding and by storm-driven waves. Beach sediment is sorted, rounded, winnowed and transported alongshore by waves to fi ll the beach embayment with ridges of very well sorted and well-rounded gravel. A gentlydipping shoreface platform occurs up to 100 m offshore between the beach and the brink point. From the brink point, in water depth of about 8 m, sediment cascades down the steep delta front slope to form long foresets extending to the lake fl oor. The sedimentary successions therefore wedge in the opposite direction from those of tectonically controlled basin margins where fan growth keeps pace with basin subsidence. RESUMEN. Deltas entrelazados lacustres del Cuaternario en el lago General Carrera, sur de Chile. Una serie de deltas entrelazados lacustres, pertenecientes al Cuaternario, ubicados en las costas del lago General Carrera, en el sur de Chile, se formaron como resultado de caídas en el nivel del lago. Cada delta consta de dos componentes: el abanico deltaico y la zona de embahiamiento. Cada uno de ellos tiene, a su vez, un elemento subaéreo costero y un elemento subacuático. Los abanicos deltaicos tienen una forma Gilbert clásica con una parte superior de ríos entrelazados y un frente deltaico empinado. Adyacente a los abanicos aluviales existen bahías cóncavas de playa que se formaron de manera paralela a los deltas a medida que estos progradaron hacia el lago. Los sedimentos deltaicos consisten predominantemente de gravas matriz y clastosoportadas con un grosor total compuesto que excede los 400 m. Los ambientes depositacionales están dominados por inundaciones efímeras del río y por olas de tormenta. Los sedimentos de playa están seleccionados, redondeados y fueron transportados a lo largo de la costa por las olas para rellenar el embahiamiento de la playa con dorsales de gravas muy bien clasifi cadas y redondeadas. Una plataforma del 'shoreface' de baja pendiente ocurre hasta 100 m agua adentro, entre la orilla y el borde. Desde el borde del lago, en el agua hasta los 8 m de profundidad, el sedimento cae en cascadas por la empinada pendiente del frente deltaico para formar grandes 'foresets' que se extienden hacia el fondo del lago. La sucesión sedimentaria se acuña, en consecuencia, en una dirección opuesta a la de aquella, de márgenes de cuenca controlados tectónicamente ('bri...

Gilbert‐type deltas recording short‐term base‐level changes: Delta‐brink morphodynamics and related foreset facies

“…(A) Schematic longitudinal cross‐section of a Gilbert‐type delta, depicting its characteristic tripartite architecture and other common features (compiled from Bell, ; Gobo et al ., ). The formation of such deltas reflects a high basin/river depth ratio; no scale is given, as the delta thickness depends on the basin accommodation and may range from a few metres to a few hundred metres.…”

Glacial lake drainage in Patagonia (13-8 kyr) and response of the adjacent Pacific Ocean

“…Although large meltwater lakes existed in the mid-latitudes of the Southern Hemisphere in Patagonia678, the nature of their drainage during deglaciation and the impact of this on regional climate has not been similarly investigated despite the recognition of late glacial millennial-scale climate changes in a variety of Southern Hemisphere terrestrial environments9. Patagonia, situated in the southern mid-latitudes near the core of the precipitation-bearing southern westerlies, is arguably one of the most climatically sensitive areas in the world and interpreting the environmental signatures recorded in marine and terrestrial records here is important to test models of environmental change and ice-sheet behaviour on regional to interhemispheric scales10.…”

“…At times, the two icefields coalesced to form the Patagonian Ice Sheet (PIS), which covered the Andes between latitudes 38  o S and 56  o S1213. During deglaciation a series of glacial lakes formed east of the Andes in the over-deepened basins currently occupied by Lago General Carrera/Lago Buenos Aires (LGC/LBA) and Lago Cohrane/Pueyrredón (LC/LP)678. Previous studies678 have described the glacial lake sedimentology and glacial lake outline but little is known about the lake response to PIS deglaciation, especially the organization, timing and routing of drainage events.…”

“…During deglaciation a series of glacial lakes formed east of the Andes in the over-deepened basins currently occupied by Lago General Carrera/Lago Buenos Aires (LGC/LBA) and Lago Cohrane/Pueyrredón (LC/LP)678. Previous studies678 have described the glacial lake sedimentology and glacial lake outline but little is known about the lake response to PIS deglaciation, especially the organization, timing and routing of drainage events. Here we present new geomorphological mapping and 16 new OSL dates that provide chronological constraints on major glacial lake nature and evolution of the region.…”

The glacial geomorphology of the Lago Buenos Aires and Lago Pueyrredón ice lobes of central Patagonia

“…Flat-topped, sediment accumulations in the mouths of tributary valleys, and perched above modern lakes, represent raised lacustrine deltas (Figure 13; Bell, 2008Bell, , 2009Glasser et al, 2016;Hein et al, 2010;Turner et al, 2005). At LGC-BA, raised deltas are often flanked by beaches (Figure 13(D)).…”