关键词: 盆地结构, 构造演化, 古近系, 北部坳陷

Abstract: The Paleogene NE-NEE-trending faults control the structural framework of the northern depression, Beibuwan Basin, and
the NEE- and NW-trending faults control the development of the sub-structural units. As a result, a NEE-trending half-graben basin
was formed, which faulted in the northwestern of the basin and overlapped in the southeastern of the basin. The basin structures of
the Weixinan sag and the Haizhong sag show characteristics of complicated half-graben and simple half-graben, respectively, and the
Weixinan low uplift is controlled by a boundary fault. The Paleogene evolution of the study area can be divided into two stages, i.e.,
rifting during the depositional period from the Changliu Formation to the Wei-3 Member, and transitional from rifting to depression
during the depositional period from the Wei-2 Member to the Wei-1 Member. The rifting stage can be further subdivided into rifting
I episode (Changliu Formation to Liu-2 Member) and rifting II episode (Liu-1 Member to Wei-3 Member). The sedimentary features
of the study area have response to the basin structure and tectonic evolution. During the rifting I episode, the tectonic activity in the 
 study area was intense and the sediments covered the entire lake to form a basin, which developed various sedimentary facies
including nearshore subaqueous fan in steep slope, braided deltas in gentle slope, and lake and sublacustrine fan in subsag.
During the rifting II episode, the tectonic activity was subdued and the Weixinan low uplift was exposed to the surface which
separates the Weixinan sag from the Haizhong sag, and various sedimentary facies were formed including nearshore subaqueous fan
and fan deltas in steep slope, large braided deltas in gentle slope, and lake or sublacustrine fan in sub-sag. During the transitional
stage, the tectonic activity was weak, which was dominated by shallow lacutrine facies, and plenty of braided deltas were developed
in steep and gentle slopes, whereas the Weixinan low uplift was still exposed to the surface.

Key words: basin structure, structural evolution, Paleogene, northern depression