Georgia O'Keeffe primarily used charcoal for her 1916 abstract drawings.
The works were made while she was teaching art in Canyon, Texas, and they reduced landscapes and other visual experiences to flowing lines, shapes, and tonal contrasts. O'Keeffe sent some of these drawings to her friend Anita Pollitzer, who showed them to Alfred Stieglitz.
Stieglitz exhibited the drawings at his 291 gallery in New York in 1916. Their abstract character was striking because they were not conventional studies of identifiable objects. They helped establish O'Keeffe as an important emerging modernist before her later flower and New Mexico paintings became widely known.
O'Keeffe also worked extensively in watercolor and oil, so medium can be a source of confusion. The question refers specifically to the influential 1916 abstract drawing series, whose main medium was charcoal on paper.