GPR STRATIGRAPHY AND QUATERNARY MORPHOGENESIS IN THE SEMIARID BRAZIL

Abstract The Serra da Capivara National Park is an optimal case study for Quaternary and geomorphology studies in the semiarid region of northeastern Brazil. Previous research in the Serra Branca valley, which is one of the park’s main geomorphological units, reveals the predominance of thick colluvial deposits, suggesting episodic fluctuations towards wetter semiarid climates since the Upper Pleistocene. This study focuses on the application of GPR stratigraphy to Quaternary deposits along the three kilometers-wide middle reach of the Serra Branca valley. High-resolution GPR and RTK data and OSL dating suggest a more complex geomorphic evolution than previously envisioned. Results indicate the existence of gullied first-order watersheds that lie buried under small alluvial fan deposits since at least the Early to Middle Holocene. A 300 meters-wide alluvium filled paleo-channel is found, hanging about 14 meters higher than the present valley bottom and recording the incision of the valley since the Last Glacial Maximum. GPR and RTK data enable mapping truncated rock surfaces; colluvial and alluvial deposits; pediments; worn down watersheds; a relict fluvial channel, and the present valley bottom. Results suggest that the local climate during the last global glacial stage was wetter than today.

摘要 卡皮瓦拉山国家公园（Serra da Capivara National Park）是巴西东北部半干旱区域开展第四纪与地貌学研究的理想案例区。此前针对该公园核心地貌单元之一的塞拉布兰卡河谷（Serra Branca valley）开展的前期研究显示，区域内厚层坡积物占据主导，表明自晚更新世以来，半干旱气候曾出现向湿润化方向的阶段性波动。本研究将探地雷达（Ground Penetrating Radar, GPR）地层学方法应用于塞拉布兰卡河谷3公里宽中段的第四纪沉积体系。高分辨率探地雷达、实时动态差分定位（Real-time Kinematic, RTK）数据与光释光（Optically Stimulated Luminescence, OSL）测年结果表明，该区域的地貌演化过程较此前学界的预想更为复杂。研究结果显示，至少自早全新世至中全新世时期，小型冲积扇沉积层之下便掩埋了一级冲沟流域。本次研究发现一处宽300米的充填型古河道，其高程较现今河谷谷底高出约14米，完整记录了自末次盛冰期以来河谷的下切演化历史。探地雷达与RTK数据可用于精准绘制被截断的岩石露头面、坡积与冲积沉积体、山麓剥蚀面、侵蚀退化的流域、残留古河道以及现今河谷谷底的空间分布。研究结果同时表明，末次全球冰期阶段的当地气候相较于现今更为湿润。