Claiton Marlon dos Santos Scherer

Eolian dunes of the Botucatu Formation (Cretaceous) in southernmost Brazil: morphology and origin

Abstract The Botucatu Formation consists of eolian sandstones that cover a large area of the central portion of South America, being one of the most impressive occurrences of continental deposits globally. A detailed facies analysis was performed in several exposures of the Botucatu Formation in the Rio Grande do Sul State (southernmost Brazil). Two basic morphological types of eolian dunes were identified: (1) simple, locally compound crescentic dunes; (2) complex linear dunes. Superimposed dunes migrating obliquely to the lee slope of the main bedforms characterize the crescentic draas. Superimposed crescentic dunes migrating parallel to the draa brink compose the linear draas. The linear draas experienced important lateral migration. Both types of composed dunes are morphodynamically interpreted as oblique dunes. The eolian dunes succession of the Botucatu Formation may be defined as a dry eolian system. As super surfaces are absent within the eolian strata, the Botucatu Formation represents a single eolian accumulation. The end of the eolian sedimentation and the development of a super bounding surface were the result of the rapid extrusion of the Serra Geral basaltic lava floods, which preserved the bedforms morphology. This fact, associated with the flexural subsidence caused by the thick lava floods, allowed the preservation of the entire eolian accumulation into the geological record.

Estratigrafia de alta resolução dos estágios iniciais de rifteamento, Bacia De Sergipe-Alagoas, Brasil.

Manuscript ID: 20170003. Received on: 01/10/2017. Approved on: 08/21/2017. ABSTRACT: The present work aims to characterize the NeoJurassic to Neocomian succession of the Sergipe-Alagoas Basin, located in northeast region of Brazil, in order to discover the influence of tectonics on sedimentation in detailed scale and thus separating this sedimentary succession in tectono-stratigraphic units. Fieldwork observations and stratigraphic sections analysis allowed subdividing this rift succession into three depositional units that indicate different paleogeographic contexts. Unit I, equivalent to the top of Serraria Formation, is characterized by braided fluvial channel deposits, with paleocurrent direction to SE; unit II, corresponding to the base of Feliz Deserto Formation, is composed of anastomosed fluvial channel and floodplain facies associations; and unit III, equivalent to the major part of Feliz Deserto Formation, is characterized by delta deposits with polymodal paleocurrent pattern. The changes of depositional system, as well as paleocurrent direction, suggest that the previously described units were deposited in different evolutionary stages of rifting. Units I and II represent the record of a wide and shallow basin associated with the first stage of rifting. Unit I is characterized by incipient extensional stress generating a wide synclinal depression, associated to the low rate of accommodation and low tectonic activity. These two parameters progressively increase in unit II. The paleocurrent direction of unit I indicates that the depocenter of this wide basin was located at SE of the studied area. No conclusion could be done on paleocurrent from unit II because of the low amount of measurements. Unit III suggests a second stage marked by a deeper basin context, with a high rate of accommodation space associated with the lateral connection of faults and individualization of the half-graben. The scattering in the paleocurrent direction in this unit indicates sedimentary influx coming from several sectors of the half-graben. The boundary between these two stages is marked by a flooding surface that indicates an extremely fast transition and suggests a radical change in geometric characteristics of the basin due to the increase of tectonic activity.The present work aims to characterize the NeoJurassic to Neocomian succession of the Sergipe-Alagoas Basin, located in northeast region of Brazil, in order to discover the influence of tectonics on sedimentation in detailed scale and thus separating this sedimentary succession in tectono-stratigraphic units. Fieldwork observations and stratigraphic sections analysis allowed subdividing this rift succession into three depositional units that indicate different paleogeographic contexts. Unit I, equivalent to the top of Serraria Formation, is characterized by braided fluvial channel deposits, with paleocurrent direction to SE; unit II, corresponding to the base of Feliz Deserto Formation, is composed of anastomosed fluvial channel and floodplain facies associations; and unit III, equivalent to the major part of Feliz Deserto Formation, is characterized by delta deposits with polymodal paleocurrent pattern. The changes of depositional system, as well as paleocurrent direction, suggest that the previously described units were deposited in different evolutionary stages of rifting. Units I and II represent the record of a wide and shallow basin associated with the first stage of rifting. Unitxa0 I is characterized by incipient extensional stress generating a wide synclinal depression, associated to the low rate of accommodation and low tectonic activity. These two parameters progressively increase in unit II. The paleocurrent direction of unit I indicates that the depocenter of this wide basin was located at SE of the studied area. No conclusion could be done on paleocurrent from unit II because of the low amount of measurements. Unit III suggests a second stage marked by a deeper basin context, with a high rate of accommodation space associated with the lateral connection of faults and individualization of the half-graben. The scattering in the paleocurrent direction in this unit indicates sedimentary influx coming from several sectors of the half-graben. The boundary between these two stages is marked by a flooding surface that indicates an extremely fast transition and suggests a radical change in geometric characteristics of the basin due to the increase of tectonic activity.