High resolution stratigraphy of initial stages of rifting, Sergipe-Alagoas Basin, Brazil

ABSTRACT: The present work aims to characterize the Neo-Jurassic 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.

摘要：本研究旨在对位于巴西东北部的塞尔日皮-阿拉戈斯盆地（Sergipe-Alagoas Basin）的新侏罗纪至凡兰吟阶（Neocomian）沉积层序进行系统表征，以高精度尺度揭示构造作用对沉积作用的管控机制，并据此将该沉积层序划分为构造地层单元（tectono-stratigraphic units）。通过野外地质观测与地层剖面分析，研究人员将该裂谷沉积层序划分为三个代表不同古地理背景的沉积单元：沉积单元I对应塞拉里亚组（Serraria Formation）顶部，以辫状河道沉积为特征，古流向指向东南；沉积单元II对应费利兹德塞尔图组（Feliz Deserto Formation）底部，由网状河道与泛滥平原相组合构成；沉积单元III对应费利兹德塞尔图组主体部分，以发育多模态古流向（polymodal paleocurrent）的三角洲沉积为特征。沉积体系与古流向的变化表明，前述三个沉积单元形成于裂谷作用的不同演化阶段。单元I与单元II代表了裂谷作用第一阶段的宽浅盆地沉积记录：单元I形成于初始伸展应力作用阶段，发育宽缓向斜凹陷，伴随可容纳空间（accommodation space）增长速率低、构造活动弱的特征；上述两个参数在单元II中逐渐升高。单元I的古流向表明，该宽浅盆地的沉积中心位于研究区东南方向；由于观测数据量不足，无法从单元II的沉积中获取有效古流向结论。单元III代表裂谷作用第二阶段，对应盆地加深的沉积环境，伴随可容纳空间快速增长，同时发育断层侧向连通与半地堑（half-graben）的个体化特征。该单元古流向的分散性表明，沉积物供给来自半地堑的多个区域。两阶段的分界以海泛面（flooding surface）为标志，表明沉积环境发生了极快速的转变，同时反映构造活动增强导致盆地几何形态发生根本性变化。